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 set that as your default (vboptions > style & languages > default style) and browse the site using that totally default vbulletin style - do you still have the same problem?
Also, here is a page from the manual about troubleshooting blank or 'white' pages:
https://www.vbulletin.com/docs/html/blank_pages