If you haven't already, try to disable plugins, by editing your includes/config.php and inserting this line:
Code:
define('DISABLE_HOOKS', 1);
near the top just under the <?php line. If that stops the message then the hack is in a plugin, and you can probably go to the plugin manager and figure out which one.
If that doesn't work, try downloading and running this:
www.vbulletin.org/forum/showthread.php?t=281080