Reupload all new totally default vbulletin files. Verify this in Maintenance > Disagnostics > Suspect File Versions - none of the default files should have "Does not contain expected contents" next to them (I don't care about any non-default vbulletin files).
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?