[HMBeaty]

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?

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

PHP Code:

define('DISABLE_HOOKS', true);