Hello eekamouse,
I have already configured an experimental setup using postfix with virtual users setup and I wanted to get it up and running with a roundcube front-end. After searching the forums, I found this post and I'm just wondering if anybody was successful with implementing it. Reading through the install guides and some howtos, I noticed what you meant by which database would actually be used. Since if you use the query method, it will need to know the database to connect to. This is not possible I think, since there is only one database that is set-up from the beginning in the db.inc file. Copying the users from the postfix database is a logical suggestion, but that in itself would also pose problems. I will try this though, and see if it can allow the virtual users to log-in. Another solution I think is by using the /etc/postfix/virtuser file. But I don't think this will be possible since the setup guide i'm using from howtoforge doesn't seem to include it. I might try it though.
sicnarf