Why aren't things from /vendor being copied during bundle install? - ruby-on-rails

Question

I'm trying to install bootstrap-sass-rails. I copied this line: gem 'bootstrap-sass-rails' into the assets group. I ran bundle install. I followed the directions on the README exactly, but Sass complained that it couldn't find 'twitter/bootstrap' - I fiddled around a bit with locating the file and then...
Here comes the confusing part -- the gem was installed fine (or so Bundler said). But now there's nothing in my /vendor/assets/stylesheets directory! I checked git status and no new Sass files (or anything else really) was changed except app/assets/stylesheets/application.css.scss and app/assets/javascripts/application.js! I thought maybe that gem was broken, but the same thing happened with bootstrap-sass.
Why aren't my gems being installed correctly?
Full Gemfile:
source 'https://rubygems.org'
gem 'rails', '3.2.12'
gem 'bson_ext'
gem 'nokogiri'
gem 'execjs'
gem 'therubyracer'
gem 'rb-readline', '~> 0.4.2'
gem 'jquery-rails'
gem 'simple_form'
gem 'mysql2'
gem 'chronic'
gem 'prawn-labels'
gem 'draper'
gem 'ruby-aaws'
gem 'bcrypt-ruby'
gem 'sass-rails', '~> 3.2'
group :assets do
gem 'coffee-rails', '~> 3.2.1'
gem 'uglifier', '>= 1.0.3'
gem 'bootstrap-sass-rails'
end
group :development do
gem 'better_errors'
gem 'binding_of_caller'
gem 'quiet_assets'
gem 'thin'
end
group :development, :test do
gem 'bourne'
gem 'rspec-rails'
gem 'pry'
gem 'pry-doc'
gem 'pry-debugger'
gem 'pry-rails'
gem 'faker'
gem 'pdf-inspector'
gem 'guard-rspec'
gem 'guard-zeus'
gem 'guard-livereload'
gem 'debugger'
gem 'rb-fsevent' if RUBY_PLATFORM =~ /darwin/i
if RUBY_PLATFORM =~ /linux/i then
gem 'rb-inotify' # Filesystem changes
gem 'libnotify' # Desktop notifications
end
end
group :test do
gem 'factory_girl_rails'
end

Answer 1

For "bootstrap-sass-rails" gem, the actual js and css files are located in gem's directory. They won't be copied to /vendor. And other gems do the same unless they provide a special generate task.
These directories will be included in assets' path. So when you mention #import bootstrap, Rails know where to search the files.