I don't see the sidebar on that page, so I can't see what is wrong with your CSS.
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 don't, then compare the css on that page to the one using your style. (You can use a tool like firebug to do this.)