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 you still have the problem, you will need to post on vb.com for help. See this sticky -
Installation Issues / Questions or bug reports regarding default vBulletin 4.0