I have Rails 4rc1 application. I need to integrate some JavaScript / CSS plugins / 3rd party libraries/ to my application. I know vendor/plugins directories are deprecated i
Rails 4.0 no longer supports loading plugins from vendor/plugins. You must replace any plugins by extracting them to gems and adding them to your Gemfile. If you choose not to make them gems, you can move them into, say, lib/my_plugin/* and add an appropriate initializer in config/initializers/my_plugin.rb.
http://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html#upgrading-from-rails-3-2-to-rails-4-0-vendor-plugins
You could of course move the javascript libraries to anywhere in your app/assets/
folder.