Disable vbseo completely (through the vbseo admin panel, the products page, and the htaccess file) and see if you still have the same problem.
If so, also 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?