vagrant-cachier-ng/docs/buckets/rubygems.md

RubyGems

Compatible with probably with any type of guest distro, will hook into the cache folder under the result of running gem env gemdir as the default SSH user (usualy vagrant) on your guest. If you use rbenv / rvm on the guest machine, make sure it is already installed before enabling the bucket, otherwise you won't benefit from this plugin.

To manually enable it:

Vagrant.configure("2") do |config|
  config.vm.box = 'some-box-with-ruby-installed'
  config.cache.enable :gem
end

Heads up about bundle install --deployment

Please note that when the --deployment flag is passed on to bundle install your gems will not be cached since bundler ends up skipping the default gem cache dir. For more information about this, please check GH-62.