The current thinking is
http://www.vbulletin.com/forum/showpost.ph...617&postcount=4
http://www.vbulletin.com/forum/showthread.php?t=116398
Since you're not preserving a theme, you simplify things greatly.
What I would recommend is staging it to make sure. i.e.
http://vb.oprano.com, separate database, etc. Do the import, check things, double check things, make sure those permissions are set right, get your template designed.
It'll take you a week or so banging on it to get everything worked out -- keep copious notes.
On your switchover day, shut down invision, dump, import, turn up vb, run through your staging checklist, turn it live.
If you have interesting permission combinations, you'll want to doublecheck those, i.e. one person able to do admin in a particular group, but not in other groups, etc.