U can ... if you have the appropriate number of licenses, you can have them in the one DB by changing the table_prefix in config.php ... (as I understand) ... just make sure your hacks etc are all fully compatible by ensuring they all have " . TABLE_PREFIX ."tablename in any queries etc.
I really would prefer not to have all forums in one database. In otherwords each forum would have it's own database; however, each of those forums would have the ability to point to a one user database. I think the vb config would need a new variable $userdb = 'xyz', then everywhere a call is made to the user database it would need to utilize the $userdb value. I'm not a programmer so I don't know the scope of this potential hack.
Yep, one site going live this week and another going live later this month. Its realy easy but I would only recommend a setup like this for the serious sites.