The data structures will be the same.  The idea is that for single users to use VistaDB and for multi users then use MS-SQL Express.

I use the DDT for all my data design to take advantage of updating customer structure and data with DDT.  I have not try VistaDB yet, but will do so next week.

I also need to check if the RBS will work with VistaDB.  I will keep all my SQL commands as simple as possible to keep avoid database incompatibility, so will try to test everything in VistaDB first.  I have many single users with this application in VFP and VistaDB looks like a good alternative to MS-SQL in this case.

I only hope I could use the DDT for VistaDB since all my BOs are mapped to the DDT excepts for views which could be a problem between VistaDB and MS-SQL.  Will start testing next week with VistaDB and would like to hear from Trent or Dustin about any possible issues with this scenario and RBS compatibility issues.

Your BOs should be no problem at all. As Greg mentioned, you will just be dealing with your DataSourceKey, the BO logic shouldn't be affected, especially if you write your Queries to be compatible with both MS-SQL and VistaDB from the get-go. The DDT, however, will not be able to deploy to VistaDB, and the RBS would be sketchy at best.

The best option for your VistaDB boxes may be to put RBS on it's own SQLExpress instance. That way you don't have to worry about compatibility, and the RBS won't hit any size limits like you may run into with your main APP database. If you did take that approach, I would probably go ahead and seperate out your RBS tables into a seperate DB for all of your installations, regardless of whether they use VistaDB or MS-SQL. That way, you don't have to account for anything fancy on the security side of things, it will always be it's own DataSourceKey, which you set up once and, afterwhich, can rely on.  If you end up running into performance issues by having two connections to the same DB open for each MS-SQL client, we can re-evaluate that, but I think I'd design it simple up front, and then only add extra complication if it proves necessary after testing.

From there, the only thing you have to account for is deploying your VistaDB, and keeping track of which datasource to use for your BOs (which wouldn't be terribly hard to account for using an inherited BaseBO.)