Board Thread:Dragon City Gameplay Questions & Answers/@comment-82.23.132.243-20130625202150/@comment-1187731-20130629150714

The formular for Two Headed is Tropical + Dark, not Cactus. Since Deus Vault work in a special way so if you didn't use correct dragon (not element), you can't get Special.

After buy the formule for Great White, I tested with Uncle Sam (Electric/Metal) and Ice but never success. After that I change to Battery and get three success in straight line (if success rate isn't 100% then I'm super lucky).

Try to use Cactus, if you can't get Two Headed, then we can believe the Deus Vault process is removed (though I still it's unfair if they still keep the process).