flyguy,
Keep in mind that VistaDB is an in-memory database, so if IIS gets restarted or your appPool recycles, then a good bit of it needs to get put back into ram. We do believe that SQL Server would be your best performing option, but there are plenty of people that use VistaDB as their main database (for instance, the VistaDB blog site).
Also remember that switching between the supported databases with Graffiti is pretty simple with the DataMover utility.
Hope that helps,
David L. Penton
Telligent