Bundler ships a command which explicitly creates this cache
bundle package
After you've done this bundle install will check and keep the vendor/cache directory up to date.
If you then want to install the gems on a machine without checking on rubygems, you just run
bundle install --local
However, beware, if you are upgrading gems frequently (like I do every time a subrelease of Rails comes out), your vendor/cache can quickly grow.
My current project's git repository is 80mb, of which more than 30mb is data stored in vendor/cache.
It seemed like a good idea to speed up deploys,
but overall its just made our repository much bigger.