Marek Kowalcze

If the command can't find your app.rb file, make sure it's in your LOAD_PATH:

bundle exec irb -I. -r app.rb

Since Vagrant 1.1 this is now default behavior.
See https://github.com/mitchellh/vagrant/issues/713#issuecomment-13201507

Just for reference, for Vagrant 1.2.2 customizing provider is now done by:

config.vm.provider "virtualbox" do |v|
  v.customize (...)
end

Unfortunately it doesn't work with JRuby.