execute commands as user after Vagrant provisioning

You should be able to do this using the Vagrant Shell provisioner, e.g.

Vagrant.configure("2") do |config|
  $script = <<-SCRIPT
  rbenv install 2.0.0-p353
  rbenv global 2.0.0-p353
  gem update --system
  yes | gem update
  gem install rdoc
  gem install rails pg
  SCRIPT

  config.vm.provision "shell", inline: $script, privileged: false
end

The key is to specify privileged: false so that it will use the default user and not root.