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? If not, I would guess it is a problem with your custom FORUMHOME template (or header, headinclude, navbar, or footer template)