I saw scripts to do this a few years ago while I was looking... they exist... to tell you the truth... I have my suspicions that vbulletin is a branch of phpBB... so chances are the database structure should be similar... This seems to suggest it could be a problem... http://www.photopost.com/forum/showthread.php?t=121979 if you have to resort to writing patches yourself to transfer the info from one to the other... forget it... it's not worth it... even if someone pays you a fortune it will suck the last bit of life from you...
go to Vbulletin Pre-Sales questions, they will help you out. http://www.vbulletin.com/forum/forumdisplay.php?f=47