diff --git a/core/lib/Drupal/Core/Composer/Composer.php b/core/lib/Drupal/Core/Composer/Composer.php
index bdb0127c46..fc1fe13e50 100644
--- a/core/lib/Drupal/Core/Composer/Composer.php
+++ b/core/lib/Drupal/Core/Composer/Composer.php
@@ -72,32 +72,35 @@ class Composer {
    * Add vendor classes to Composer's static classmap.
    */
   public static function preAutoloadDump(Event $event) {
+    // Get the configured vendor directory.
+    $vendor_dir = $event->getComposer()->getConfig()->get('vendor-dir');
+
     // We need the root package so we can add our classmaps to its loader.
     $package = $event->getComposer()->getPackage();
     // We need the local repository so that we can query and see if it's likely
     // that our files are present there.
     $repository = $event->getComposer()->getRepositoryManager()->getLocalRepository();
     // This is, essentially, a null constraint. We only care whether the package
-    // is present in vendor/ yet, but findPackage() requires it.
+    // is present in the vendor directory yet, but findPackage() requires it.
     $constraint = new Constraint('>', '');
     // Check for our packages, and then optimize them if they're present.
     if ($repository->findPackage('symfony/http-foundation', $constraint)) {
       $autoload = $package->getAutoload();
       $autoload['classmap'] = array_merge($autoload['classmap'], [
-        'vendor/symfony/http-foundation/Request.php',
-        'vendor/symfony/http-foundation/ParameterBag.php',
-        'vendor/symfony/http-foundation/FileBag.php',
-        'vendor/symfony/http-foundation/ServerBag.php',
-        'vendor/symfony/http-foundation/HeaderBag.php',
+        $vendor_dir . '/symfony/http-foundation/Request.php',
+        $vendor_dir . '/symfony/http-foundation/ParameterBag.php',
+        $vendor_dir . '/symfony/http-foundation/FileBag.php',
+        $vendor_dir . '/symfony/http-foundation/ServerBag.php',
+        $vendor_dir . '/symfony/http-foundation/HeaderBag.php',
       ]);
       $package->setAutoload($autoload);
     }
     if ($repository->findPackage('symfony/http-kernel', $constraint)) {
       $autoload = $package->getAutoload();
       $autoload['classmap'] = array_merge($autoload['classmap'], [
-        'vendor/symfony/http-kernel/HttpKernel.php',
-        'vendor/symfony/http-kernel/HttpKernelInterface.php',
-        'vendor/symfony/http-kernel/TerminableInterface.php',
+        $vendor_dir . '/symfony/http-kernel/HttpKernel.php',
+        $vendor_dir . '/symfony/http-kernel/HttpKernelInterface.php',
+        $vendor_dir . '/symfony/http-kernel/TerminableInterface.php',
       ]);
       $package->setAutoload($autoload);
     }