You're viewing the documentation for the experimental version of Vagrant (the 1.x series). The current stable series is 1.0.x and the documentation can be found here. You can learn more about the Vagrant versioning scheme here.
Plugins are written using Ruby code, and anything but the most trivial plugins are distributed using RubyGems. Creating a Vagrant plugin requires a small amount of Ruby knowledge, but most of it can be picked up along the way as long as you’re programmed before.
If you’re writing your first plugin, I recommend writing it directly in a Vagrantfile, so that you don’t have to worry about RubyGems. If you’re looking to distribute your plugin, the documentation covers the entire RubyGems packaging process for you.
Plugins are composed of a number of parts:
To get started, write your first plugin definition. After this, you can either extend your plugin with full-fledged commands, provisioners, etc. or you can start with an easy plugin to get something working much more quickly.