What I've done is have my root clean, just in case I want to install something and created a folder called forums. When a user goes into the root, he's automatically redirected to
www.root.com/forums.
Whats you can also do is install vBadvanced (a mod) which has index.php as a splash page and then forums.php as your forum, in the root.
I don't know if it'll work, is there even an option to change in vBulletin options?