Well here's how I see it... who owns the hosting? If he does then leave the site and let him be, pick out another domain name and setup vB w/ your valid license, after you make the new forum update your info on vb.com and report his site as illegally using vBulletin. I would if you own it and can access the DB make a backup NOW.
Secondly you can find the admincp even if he renamed it

. Also you can IF you have to move to a new host etc copy that site pretty much unless it uses a paid custom style (then who's name is that in you see?) that requires purchasing of a multi-site license etc.
*Just some tips, I personally do not get involved in situations where all info is not provided due to suspected bias i.e. "drama" for short.