In 5th gen, the female parent has to have the Hidden Ability to pass it down, and in 6th gen, it works with either parent. Either way, you have to have one with the ability Drought. You can get a Drought Vulpix in the Dream World (which unfortunately I think it is down) if you are playing B(2) or W(2). If you are playing X or Y, you can a Drought Ninetales through the Friend Safari.