`vagarnt gem` cannot be called within a bundler env

lib/vagrant/command/gem.rb

@@ -5,6 +5,16 @@ module Vagrant
   module Command
     class Gem < Base
       def execute
+        # Bundler sets up its own custom gem load paths such that our
+        # own gems are never loaded. Therefore, give an error if a user
+        # tries to install gems while within a Bundler-managed environment.
+        if defined?(Bundler)
+          require 'bundler/shared_helpers'
+          if Bundler::SharedHelpers.in_bundle?
+            raise Errors::GemCommandInBundler
+          end
+        end
+
         # We just proxy the arguments onto a real RubyGems command
         # but change `GEM_HOME` so that the gems are installed into
         # our own private gem folder.

lib/vagrant/errors.rb

@@ -173,6 +173,11 @@ module Vagrant
       error_key(:environment_locked)
     end
 
+    class GemCommandInBundler < VagrantError
+      status_code(71)
+      error_key(:gem_command_in_bundler)
+    end
+
     class HomeDirectoryMigrationFailed < VagrantError
       status_code(53)
       error_key(:home_dir_migration_failed)

templates/locales/en.yml

@@ -47,6 +47,12 @@ en:
         may run at any given time to avoid problems with VirtualBox inconsistencies
         occurring. Please wait for the other instance of Vagrant to end and then
         try again.
+      gem_command_in_bundler: |-
+        You cannot run the `vagrant gem` command while in a bundler environment.
+        Bundler messes around quite a bit with the RubyGem load paths and gems
+        installed via `vagrant gem` are excluded by Bundler.
+
+        Instead, please include your Vagrant plugins in your Gemfile itself.
       guest:
         invalid_class: |-
           The specified guest class does not inherit from `Vagrant::Guest::Base`.