I have never used phpnuke, though I have heard that there was a vbulletin module for it released in the past. It did not turn vb into a module but it did at least merge the db's for you.
I personally use postnuke and we wrote our own vbulletin module for it so I know it's possible. Believe me when I say there is a whole lot more to it than just syncing the db's. First of all vb uses md5+salt where php does not use salt. Also what do you want the system to do when a user changes their profile info such as their password? Now you need to think about synching all the user and admin functions to phpnuke as well.
I have no plans of turning vb into a phpnuke module yet, but you can see vb as a true postnuke module at my site. Expect the code to be released late this summer as we still need to build all the db user import scripts and a few other mods.
www.annointed.net