i can see the error when clicking on the second link. A non-logged in user should just get a screen to login.
Did you ever try disabling your plugins to see if the error goes away? Or try using a default style to see if you still get the error? 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
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
define('disable_hooks', true); don't forget that for vbseo, disabling the modification means to also get rid of anything extra you added to your htaccess file.