First you need to find the problem. Make sure you are being hacked and its not one of your other admins screwing around trying to feel important.
Depending on how the hack was preformed you could loose total control of your server, until you get in touch with your hosting company or you could simply lose vBulletin.
either way make sure to have some backups, and when you restore, disable all plugins and hacks you've ever installed to make sure your vBulletin is secure, oh and using the latest version always helps