Create a new style with no parent:
Styles & Templates > Style Manager > Add New Style :
Parent Style: No Parent Style
Title: Default vBulletin
Allow User Selection: Yes
Save
Then browse the site using that totally default vbulletin style - do you still have the same problem?
I tried that but it didn't work.
One of my co-admins installed vBSEO last night and Im thinking he may have accidentally deleted some files which is causing this. What files could I check to make sure that all the files that need to be there are present?