[Opserty]

Hmm its a bit complicated but you can do it using the default vB system.

1. Create a new usergroup e.g. "Welcomed Users" with all the permissions of the "Registered Users" usergroup
2. Move all your Users that are currently in the "Registered Users" usergroup into the new group (See: Prune / Move Users)
3. Change the forum permissions for the "Registered Users" usergroup so that they can only post in the "Welcome Forum" (also remove their ability to post in other peoples threads in the "Welcome Forum" ) (See: Managing Forum Permissions)
4. Create a Usergroup Promotion to promote users from "Registered Users" to the "Welcomed Users" usergroup after they have made 1 post. (See: Usergroup Promotions)
5. Your done!