Create a new style with no parent - Styles & Templates > Style Manager > Add New Style > no parent - then login to the site using that totally default vbulletin style - do you still have the same problem?
Since it works with the default style, this means you have an error in your site style. You will have to go through the changed templates and figure out which one is causing the problem and fix it. You may want to try to use an html validator to see if it picks up on anything.