diff --git a/src/Index/ProjectIndex.php b/src/Index/ProjectIndex.php
index 8adc938..af980f8 100644
--- a/src/Index/ProjectIndex.php
+++ b/src/Index/ProjectIndex.php
@@ -3,6 +3,8 @@ declare(strict_types = 1);
 
 namespace LanguageServer\Index;
 
+use function LanguageServer\getPackageName;
+
 /**
  * A project index manages the source and dependency indexes
  */
@@ -22,10 +24,11 @@ class ProjectIndex extends AbstractAggregateIndex
      */
     private $sourceIndex;
 
-    public function __construct(Index $sourceIndex, DependenciesIndex $dependenciesIndex)
+    public function __construct(Index $sourceIndex, DependenciesIndex $dependenciesIndex, \stdClass $composerJson = null)
     {
         $this->sourceIndex = $sourceIndex;
         $this->dependenciesIndex = $dependenciesIndex;
+        $this->composerJson = $composerJson;
         parent::__construct();
     }
 
@@ -43,8 +46,8 @@ class ProjectIndex extends AbstractAggregateIndex
      */
     public function getIndexForUri(string $uri): Index
     {
-        if (preg_match('/\/vendor\/([^\/]+\/[^\/]+)\//', $uri, $matches)) {
-            $packageName = $matches[1];
+        $packageName = getPackageName($uri, $this->composerJson);
+        if ($packageName) {
             return $this->dependenciesIndex->getDependencyIndex($packageName);
         }
         return $this->sourceIndex;
diff --git a/src/Indexer.php b/src/Indexer.php
index df9655a..34ad618 100644
--- a/src/Indexer.php
+++ b/src/Indexer.php
@@ -59,6 +59,11 @@ class Indexer
      */
     private $composerLock;
 
+    /**
+     * @var \stdClasss
+     */
+    private $composerJson;
+
     /**
      * @param FilesFinder       $filesFinder
      * @param string            $rootPath
@@ -77,7 +82,8 @@ class Indexer
         DependenciesIndex $dependenciesIndex,
         Index $sourceIndex,
         PhpDocumentLoader $documentLoader,
-        \stdClass $composerLock = null
+        \stdClass $composerLock = null,
+        \stdClass $composerJson = null
     ) {
         $this->filesFinder = $filesFinder;
         $this->rootPath = $rootPath;
@@ -87,6 +93,7 @@ class Indexer
         $this->sourceIndex = $sourceIndex;
         $this->documentLoader = $documentLoader;
         $this->composerLock = $composerLock;
+        $this->composerJson = $composerJson;
     }
 
     /**
@@ -109,10 +116,11 @@ class Indexer
             $source = [];
             /** @var string[][] */
             $deps = [];
+
             foreach ($uris as $uri) {
-                if ($this->composerLock !== null && preg_match('/\/vendor\/([^\/]+\/[^\/]+)\//', $uri, $matches)) {
+                $packageName = getPackageName($uri, $this->composerJson);
+                if ($this->composerLock !== null && $packageName) {
                     // Dependency file
-                    $packageName = $matches[1];
                     if (!isset($deps[$packageName])) {
                         $deps[$packageName] = [];
                     }
@@ -174,6 +182,8 @@ class Indexer
                     if ($cacheKey !== null) {
                         $this->client->window->logMessage(MessageType::INFO, "Storing $packageKey in cache");
                         $this->cache->set($cacheKey, $index);
+                    } else {
+                        $this->client->window->logMessage(MessageType::WARNING, "Could not compute cache key for $packageName");
                     }
                 }
             }
@@ -205,7 +215,7 @@ class Indexer
                 $this->client->window->logMessage(MessageType::LOG, "Parsing $uri");
                 try {
                     $document = yield $this->documentLoader->load($uri);
-                    if (!$document->isVendored()) {
+                    if (!isVendored($document, $this->composerJson)) {
                         $this->client->textDocument->publishDiagnostics($uri, $document->getDiagnostics());
                     }
                 } catch (ContentTooLargeException $e) {
diff --git a/src/LanguageServer.php b/src/LanguageServer.php
index 035e663..3c999f2 100644
--- a/src/LanguageServer.php
+++ b/src/LanguageServer.php
@@ -187,7 +187,7 @@ class LanguageServer extends AdvancedJsonRpc\Dispatcher
 
             $dependenciesIndex = new DependenciesIndex;
             $sourceIndex = new Index;
-            $this->projectIndex = new ProjectIndex($sourceIndex, $dependenciesIndex);
+            $this->projectIndex = new ProjectIndex($sourceIndex, $dependenciesIndex, $this->composerJson);
             $stubsIndex = StubsIndex::read();
             $this->globalIndex = new GlobalIndex($stubsIndex, $this->projectIndex);
 
@@ -206,6 +206,8 @@ class LanguageServer extends AdvancedJsonRpc\Dispatcher
                 // Find composer.json
                 if ($this->composerJson === null) {
                     $composerJsonFiles = yield $this->filesFinder->find(Path::makeAbsolute('**/composer.json', $rootPath));
+                    sortUrisLevelOrder($composerJsonFiles);
+
                     if (!empty($composerJsonFiles)) {
                         $this->composerJson = json_decode(yield $this->contentRetriever->retrieve($composerJsonFiles[0]));
                     }
@@ -214,6 +216,8 @@ class LanguageServer extends AdvancedJsonRpc\Dispatcher
                 // Find composer.lock
                 if ($this->composerLock === null) {
                     $composerLockFiles = yield $this->filesFinder->find(Path::makeAbsolute('**/composer.lock', $rootPath));
+                    sortUrisLevelOrder($composerLockFiles);
+
                     if (!empty($composerLockFiles)) {
                         $this->composerLock = json_decode(yield $this->contentRetriever->retrieve($composerLockFiles[0]));
                     }
@@ -230,7 +234,8 @@ class LanguageServer extends AdvancedJsonRpc\Dispatcher
                     $dependenciesIndex,
                     $sourceIndex,
                     $this->documentLoader,
-                    $this->composerLock
+                    $this->composerLock,
+                    $this->composerJson
                 );
                 $indexer->index()->otherwise('\\LanguageServer\\crash');
             }
@@ -252,7 +257,8 @@ class LanguageServer extends AdvancedJsonRpc\Dispatcher
                     $dependenciesIndex,
                     $sourceIndex,
                     $this->composerLock,
-                    $this->documentLoader
+                    $this->documentLoader,
+                    $this->composerJson
                 );
             }
 
diff --git a/src/PhpDocument.php b/src/PhpDocument.php
index b838cd3..3a25c23 100644
--- a/src/PhpDocument.php
+++ b/src/PhpDocument.php
@@ -220,17 +220,6 @@ class PhpDocument
         }
     }
 
-    /**
-     * Returns true if the document is a dependency
-     *
-     * @return bool
-     */
-    public function isVendored(): bool
-    {
-        $path = Uri\parse($this->uri)['path'];
-        return strpos($path, '/vendor/') !== false;
-    }
-
     /**
      * Returns array of TextEdit changes to format this document.
      *
diff --git a/src/Server/TextDocument.php b/src/Server/TextDocument.php
index 624bc67..aa76ec2 100644
--- a/src/Server/TextDocument.php
+++ b/src/Server/TextDocument.php
@@ -30,7 +30,7 @@ use LanguageServer\Index\ReadableIndex;
 use Sabre\Event\Promise;
 use Sabre\Uri;
 use function Sabre\Event\coroutine;
-use function LanguageServer\waitForEvent;
+use function LanguageServer\{waitForEvent, isVendored};
 
 /**
  * Provides method handlers for all textDocument/* methods
@@ -134,7 +134,7 @@ class TextDocument
     public function didOpen(TextDocumentItem $textDocument)
     {
         $document = $this->documentLoader->open($textDocument->uri, $textDocument->text);
-        if (!$document->isVendored()) {
+        if (!isVendored($document, $this->composerJson)) {
             $this->client->textDocument->publishDiagnostics($textDocument->uri, $document->getDiagnostics());
         }
     }
@@ -409,9 +409,9 @@ class TextDocument
                 $symbol->$prop = $val;
             }
             $symbol->fqsen = $def->fqn;
-            if (preg_match('/\/vendor\/([^\/]+\/[^\/]+)\//', $def->symbolInformation->location->uri, $matches) && $this->composerLock !== null) {
+            $packageName = getPackageName($def->symbolInformation->location->uri, $this->composerJson);
+            if ($packageName && $this->composerLock !== null) {
                 // Definition is inside a dependency
-                $packageName = $matches[1];
                 foreach (array_merge($this->composerLock->packages, $this->composerLock->{'packages-dev'}) as $package) {
                     if ($package->name === $packageName) {
                         $symbol->package = $package;
diff --git a/src/Server/Workspace.php b/src/Server/Workspace.php
index 5c0e9fa..b94618c 100644
--- a/src/Server/Workspace.php
+++ b/src/Server/Workspace.php
@@ -8,7 +8,7 @@ use LanguageServer\Index\{ProjectIndex, DependenciesIndex, Index};
 use LanguageServer\Protocol\{SymbolInformation, SymbolDescriptor, ReferenceInformation, DependencyReference, Location};
 use Sabre\Event\Promise;
 use function Sabre\Event\coroutine;
-use function LanguageServer\waitForEvent;
+use function LanguageServer\{waitForEvent, getPackageName};
 
 /**
  * Provides method handlers for all workspace/* methods
@@ -49,13 +49,14 @@ class Workspace
      * @param \stdClass         $composerLock      The parsed composer.lock of the project, if any
      * @param PhpDocumentLoader $documentLoader    PhpDocumentLoader instance to load documents
      */
-    public function __construct(ProjectIndex $index, DependenciesIndex $dependenciesIndex, Index $sourceIndex, \stdClass $composerLock = null, PhpDocumentLoader $documentLoader)
+    public function __construct(ProjectIndex $index, DependenciesIndex $dependenciesIndex, Index $sourceIndex, \stdClass $composerLock = null, PhpDocumentLoader $documentLoader, \stdClass $composerJson = null)
     {
         $this->sourceIndex = $sourceIndex;
         $this->index = $index;
         $this->dependenciesIndex = $dependenciesIndex;
         $this->composerLock = $composerLock;
         $this->documentLoader = $documentLoader;
+        $this->composerJson = $composerJson;
     }
 
     /**
@@ -122,8 +123,7 @@ class Workspace
                         $symbol->$prop = $val;
                     }
                     // Find out package name
-                    preg_match('/\/vendor\/([^\/]+\/[^\/]+)\//', $def->symbolInformation->location->uri, $matches);
-                    $packageName = $matches[1];
+                    $packageName = getPackageName($def->symbolInformation->location->uri, $this->composerJson);
                     foreach (array_merge($this->composerLock->packages, $this->composerLock->{'packages-dev'}) as $package) {
                         if ($package->name === $packageName) {
                             $symbol->package = $package;
diff --git a/src/utils.php b/src/utils.php
index 8ab7beb..c0c5bf7 100644
--- a/src/utils.php
+++ b/src/utils.php
@@ -7,6 +7,7 @@ use Throwable;
 use InvalidArgumentException;
 use PhpParser\Node;
 use Sabre\Event\{Loop, Promise, EmitterInterface};
+use Sabre\Uri;
 
 /**
  * Transforms an absolute file path into a URI as used by the language server protocol.
@@ -131,3 +132,60 @@ function stripStringOverlap(string $a, string $b): string
     }
     return $b;
 }
+
+/**
+ * Use for sorting an array of URIs by number of segments
+ * in ascending order.
+ *
+ * @param array $uriList
+ * @return void
+ */
+function sortUrisLevelOrder(&$uriList)
+{
+    usort($uriList, function ($a, $b) {
+        return substr_count(Uri\parse($a)['path'], '/') - substr_count(Uri\parse($b)['path'], '/');
+    });
+}
+
+/**
+ * Checks a document against the composer.json to see if it
+ * is a vendored document
+ *
+ * @param PhpDocument    $document
+ * @param \stdClass|null $composerJson
+ * @return bool
+ */
+function isVendored(PhpDocument $document, \stdClass $composerJson = null): bool
+{
+    $path = Uri\parse($document->getUri())['path'];
+    $vendorDir = getVendorDir($composerJson);
+    return strpos($path, "/$vendorDir/") !== false;
+}
+
+/**
+ * Check a given URI against the composer.json to see if it
+ * is a vendored URI
+ *
+ * @param \stdClass|null $composerJson
+ * @param string         $uri
+ * @param array          $matches
+ * @return string|null
+ */
+function getPackageName(string $uri, \stdClass $composerJson = null)
+{
+    $vendorDir = str_replace('/', '\/', getVendorDir($composerJson));
+    preg_match("/\/$vendorDir\/([^\/]+\/[^\/]+)\//", $uri, $matches);
+    return $matches[1] ?? null;
+}
+
+/**
+ * Helper function to get the vendor directory from composer.json
+ * or default to 'vendor'
+ *
+ * @param \stdClass|null $composerJson
+ * @return string
+ */
+function getVendorDir(\stdClass $composerJson = null): string
+{
+    return $composerJson->config->{'vendor-dir'} ?? 'vendor';
+}
diff --git a/tests/PhpDocumentTest.php b/tests/PhpDocumentTest.php
index b9b3704..011a231 100644
--- a/tests/PhpDocumentTest.php
+++ b/tests/PhpDocumentTest.php
@@ -12,6 +12,7 @@ use LanguageServer\ContentRetriever\FileSystemContentRetriever;
 use LanguageServer\Protocol\{SymbolKind, Position, ClientCapabilities};
 use LanguageServer\Index\{Index, ProjectIndex, DependenciesIndex};
 use PhpParser\Node;
+use function LanguageServer\isVendored;
 
 class PhpDocumentTest extends TestCase
 {
@@ -42,18 +43,18 @@ class PhpDocumentTest extends TestCase
     public function testIsVendored()
     {
         $document = $this->createDocument('file:///dir/vendor/x.php', "<?php\n$\$a = new SomeClass;");
-        $this->assertEquals(true, $document->isVendored());
+        $this->assertEquals(true, isVendored($document));
 
         $document = $this->createDocument('file:///c:/dir/vendor/x.php', "<?php\n$\$a = new SomeClass;");
-        $this->assertEquals(true, $document->isVendored());
+        $this->assertEquals(true, isVendored($document));
 
         $document = $this->createDocument('file:///vendor/x.php', "<?php\n$\$a = new SomeClass;");
-        $this->assertEquals(true, $document->isVendored());
+        $this->assertEquals(true, isVendored($document));
 
         $document = $this->createDocument('file:///dir/vendor.php', "<?php\n$\$a = new SomeClass;");
-        $this->assertEquals(false, $document->isVendored());
+        $this->assertEquals(false, isVendored($document));
 
         $document = $this->createDocument('file:///dir/x.php', "<?php\n$\$a = new SomeClass;");
-        $this->assertEquals(false, $document->isVendored());
+        $this->assertEquals(false, isVendored($document));
     }
 }