Try this - create a new style with no parent - Styles & Templates > Style Manager > Add New Style > no parent - then browse the site using that totally default vbulletin style - do you still have the same problem?
Then go to edit the MEMBERINFO template and then hit View History and Compare. You will then see the differences between your template and the vbulletin one so you know what to change.
If you don't have any modifications installed on that page, you could probably just hit Revert for the MEMBERINFO template. But, you can't undo that, so you may want to copy the contents of the template into a text editor first.