Does it show up on a vbulletin default style (you only show your custom style)? Are all your files totally default? Have you looked at your error_logs to see if anything is in there (if you don't know where they are, ask your host)?
How to create a vbulletin default style:
- 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?