[calorie]

You could add a new required profile field and moderate new members:

• vB ACP -> User Profile Fields -> Add New User Profile Field -> Multiple-Line Text Box
• vB ACP -> vBulletin Options -> User Registration Options -> Moderate New Members

Alternatively, you could set up a new joinable usergroup allowed to post:

• vB ACP -> Usergroups -> Add New Usergroup -> Public (Joinable) Custom Usergroups
• Remove posting ability from other usergroups and moderate users via Join Requests