[ArbuZz]

This error:

Fatal error: Call to a member function query_first_slave() on a non-object in [some path removed]Public_html\modules\forum\includes\functions.php on line 1270

has something with the global $db object. Probably you have one defined somewhere globally, and vBulletin uses another global $db of it's own. So that the overlap and conflict. You can try to rename your $db object to $db2 for example and try again. I wanted to try, but had no spare time to do this. So I cann't say for sure whether it will work or no. Also, vBulletin uses a lot of other global variables that may mess up with your own ones. There are guys that recommend to place vBulletin installation in different location and interact with it through the cURL, to totally isolate the namespaces. By now I think similarly.