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?
If it is still happening - Try disabling your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit config.php and add this line right under <?php