In OS packages ruby2 comes embedded, but if you install vagrant from sources, you should execute vagrant with ruby2.

At least, with ruby 1.9.1 and using docker as a provisioner, it gives the error:

/var/lib/gems/1.9.1/gems/vagrant-1.4.1.dev/plugins/provisioners/docker/plugin.rb:13:in `require_relative': /var/lib/gems/1.9.1/gems/vagrant-1.4.1.dev/plugins/provisioners/docker/config.rb:23: syntax error, unexpected tPOW (SyntaxError)
      def run(name, **options)
Merge pull request #2661 from adrianlzt/feature/readme_ruby2_needed

README: If using from sources, ruby2 is needed