I seem to keep finding new solutions for collaborating in Vim. The latest is Floobits by Geoff Greer and Matt Kaniaris. It uses a server, on which you must create an account, and a Vim plugin that allows you to collaborate with other people. There is also a Sublime Text plugin -- by using a central server the authors have managed to make an editor agnostic solution.
Although the plugins are naturally open source, the server doesn't appear to be. It's free to use and you can sign in with GitHub, and there is API documentation should you wish to write other clients or plugins for the service.
Once you've signed in to Floobits, you'll need to create some rooms. Rooms allow files and buffers to be grouped together, so you'll typically use a single room per-project.
Floobits also supports Google Hangouts -- Floobits can be viewed from a hangout by using the Floobits web editor.
The Vim plugin needs an editor compiled with client server support -- if you've got MacVim it should work out of the box.