vbulletin is not free.
In order to accomplish this you will need to add some custom coding. Basically when a user comes to the forum and tries to register you will need to redirect them to your regular registration page (this is how we do it as well) and when you add them to your database, you will also need to add them to the forum software database with the specified information.
You can use DataAssist insert server behavior when adding the user to the forum database. or just write your own code to do that. And after a user log in, you will need to set the appropriate cookies and session to log them automatically to the forum.