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?
If it's still happening, reupload all the original vB non-image files (except install.php). Make sure you upload these in ASCII format and overwrite the ones on the server.