What version of PHP are you running and what version on PCRE (all on the phpinfo page under maintenance).
If you still have the same problem after disabling your plugins (Devin's suggestion above), then 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 and see if you still have the same problem.