As of Media Browser 2.2.1 (codename Firestorm) the Vanilla theme is no longer part of the application.
This theme is now a plugin which is currently under development by Throgmorton.
The Vanilla theme has also been rebranded to Harmony.
More details about the progress of this plugin can be found on http://www.throgsoft.com
More details about this theme and others can be found here:
What themes and plugins exist?