The following guides and best practices should get you up and running again. Be thorough when using them, and sorry you were hacked.
Then please read the following two blog posts:
http://www.vbulletin.com/forum/blogs...ve-been-hacked
http://www.vbulletin.com/forum/blogs...vbulletin-site
Also please see these recent security announcements:
http://www.vbulletin.com/forum/forum...-1-vbulletin-5
http://www.vbulletin.com/forum/forum...d-all-versions