To include it with subsonic I think you would have to work out a deal with VistaDB. I just looked at the license more carefully... the main thing is you can't distribute the runtime. And to develop with the runtime I think they want you to have a license. Anyway, the provider is available if anyone wants to try it out. I actually am not using VistaDB for performance reasons right now... and the license also prohibits publishing benchmarks. Works great for no hassle installs and small databases, but if you have a serious amount of rows and expect to get any real work done it might not work out for you.
VistaDB, section of the license that applies:
4.1 Retail Editions of Software
Licensee may not: distribute Developed Software source code (your code) in whole or in part without acquiring additional licenses of Retail Editions of Software for each copy of Developed Software distributed, if Developed Software source code relies on Software for compilation or runtime execution.
If the Licensee distributes or offers the source code to their Developed Application they must not include the VistaDB runtime. Each developer using Developed Application must acquire a developer license in order to develop against VistaDB runtimes.
4.2 Trial Editions of Software
Licensee may not distribute, sell or rent applications or web services in whole or in part, for commercial, shareware, freeware or government use.