Places the gem binaries in vendor/cache, as noted. SCM those.
"While installing gems, Bundler will check vendor/cache and then your system's gems. If a gem isn't cached or installed, Bundler will try to install it from the sources you have declared in your Gemfile."
http://gembundler.com/bundle_install.html
Heroku uses this tree lookup AFAIK.
UPDATE:
For others' edification, the default heroku ruby buildpack respects vendor/cache, but will purge it in the following scenarios:
* if vendor/ruby_version exists
* if vendor/heroku/buildpack_version exists, but vendor/heroku/ruby_version does not
* if the bundler cache exists, but vendor/heroku/ruby_version file specifies a different version of ruby than the one actually being used.