for newer versions of Bundler, store the runtime

bin/vagrant

@@ -68,7 +68,7 @@ end
 # then also initialize the paths to the plugins.
 require "bundler"
 begin
-  Bundler.setup(:default, :plugins)
+  $vagrant_bundler_runtime = Bundler.setup(:default, :plugins)
 rescue Bundler::GemNotFound
   $stderr.puts "Bundler, the underlying system used to manage Vagrant plugins,"
   $stderr.puts "is reporting that a plugin or its dependency can't be found."

lib/vagrant.rb

@@ -288,7 +288,7 @@ end
 if Vagrant.plugins_enabled?
   begin
     global_logger.info("Loading plugins!")
-    Bundler.require(:plugins)
+    $vagrant_bundler_runtime.require(:plugins)
   rescue Exception => e
     raise Vagrant::Errors::PluginLoadError, message: e.to_s
   end