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 there is no problem with the default one, then it is something you changed and you should view the page in your style and look at the css via firebug and compare it to the page in the default, looking at the css in firebug, and hopefully you can see what was changed.