There is a script at vB.com where you upload it to your server and it checks to see if your host can support vBulletin.
most of the free hosting sites will be able to support vBulletin forums. Just find free hosting with PHP 4.3.3 or greater and MySQL 4.0.16 or greater! Have a great day.
Most will be able to support it but some will delete it assuming it was nulled since if you can afford vBulletin you can afford hosting, I advise paid hosting, its better anyway.