Is it only mods having problems?
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
SEt users to be able to change styles (vboptions > styles) and then have them browse the site using that totally default vbulletin style - do they still have the same problem?