It looks like your CSS is messed up which is something we can't check without a test username/password and login. But, do as Joe says first and check it with a default style.
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?