Just to let you know, I actually have got it working with PostgreSql, (The elephant kind

).
Although, because vBulletin is used by thousands of users worldwide with MySQL, I would probably go with that. They don't have a problem with it, and neither should you.
FYI, Although I'm not positive exactly how a database phisically stores data, I do know you can use .sql files to pick out data from the database and transfer it universally to other databases.