not really sure what your referring to as vbulletin uses ONE GLOBAL table prefix for all its tables and thats in your vb
You simply need to install our program correctly with all the correct information to have a successful install. You stated all you changed was a table prefix and I gave you a query to update just that but if you changed lots of stuff your going to want to rerun and go through an install to update all settings. Maybe you also changed your database info to the forum. I have seen people do this alot.