you will need to create a new usergroup and give access to forums only to this usergroup (forum permissions)
http://www.vbulletin.com/docs/html/m...anaging_forum_
you will then have to create a new PROMOTION and promote newly registered users to new usergroup once they meet your posting quota
http://www.vbulletin.com/docs/html/m..._add_promotion
http://www.vbulletin.com/docs/html/m...oup_promotions