Yeah, I knew that part...I wasn't sure what the default heroku ruby buildpack did. I'm still digging into the source to see what the build process is. It's non-trivial.

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.