In your cPanel or hosting panel go to parked domains, and park your .org domain on your current site. It should tell you what you should set your name servers to then. Go to your domain control for your .org domain and set your name servers. After you have done that go to you vBulletin AdminCP > vBulletin options > Site Name/URL/Contact Details and set it to the .org after it is parked. But make sure it works and forwards to your forum.
Hope that helped