Vienna 2.5 will bring support for plugins that are extremely easy to create, but at the same time are powerful enough to tailor Vienna to your needs. At the time of writing, there are two type of plugins: Link Plugins and Script Plugins. 2.5 will ship with at least three built-in ones: Share with Facebook, Share with Evernote, Share with Twitter:
URL shortening happens automatically via bit.ly and is available to all plugins, all you need to do is a click a checkbox when you are creating yours. For more information, go read the documentation. If you have questions or feedback, there's a thread about this topic on our forum.