You have to uninstall all the mods on VB3 first. Then set back a lot of DB setting to out of the box and then move over. Try this. It's a little complicated but it does work.
vBulletin 4.x upgrade process for large vBulletin forums - vBulletin Community Forum
I tried it on my private server and much to my surprise even an idiot like me could make it work. (My server is WAMP on a retired laptop.)
vBulletin 4.x upgrade process for large vBulletin forums - vBulletin Community Forum
I tried it on my private server and much to my surprise even an idiot like me could make it work. (My server is WAMP on a retired laptop.)