If the site was hacked, follow the links below, and go through them
thoroughly.
First you need to follow the vB advisory about deleting the install folder completely off your server.
Then please read the following two blog posts and follow the directions and procedures within, to the letter:
http://www.vbulletin.com/forum/blogs...ve-been-hacked
http://www.vbulletin.com/forum/blogs...vbulletin-site