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?
Also, check your htaccess file for anything related to mod_expire or mod_headers and remove (temporarily) and then see if you have the same issue.
I'm assuming you have the cookie domain set correctly to .yoursite.com also.