Some people don't know this, but Vasari 2.5 (just released today) has an SDK and it was put together by your friends at Case. The SDK has not been posted for download just yet, but Matt Jezyk said it was OK to have you guys contact him directly if you want to get an early copy of it.
Matt can be reached at: matt dot jezyk at autodesk dot com
Once you've managed to get a copy of the SDK and look inside, you may notice a lot of similarities between this Vasari SDK and the one that ships with Revit. The main difference between the Revit SDK and this new Vasari SDK is the 8 new Vasari oriented samples found in the "Vasari (NEW)" folder.
All 8 of the new Vasari samples are available in both VB.NET and C# for your programming pleasures.
Stay tuned for a daily post outlining each one of these samples and how to use them!