Vanilla 1 is no longer supported or maintained. If you need a copy, you can get it here
There was a new addon released earlier today which turns your forum into a Gmail like invite only system, thought it'll be wise to create an official release thread.
Basically it adds an invite code to the registration page so the only way for people to register is for them to be invited by members on the boards, each member gets a certain amount of invites and they can invite whoever they want. When they run out of invites they can request more from the admin.
This way if you run a private community you can give the power to the people and not have to worry about approving accounts and all that. On the account of the invitee it says "Invited by: Username" so that way you know who invited them just incase they turn out to be a problem Check it outhttp://lussumo.com/addons/index.php?PostBackAction=AddOn&AddOnID=156