The Vivi's break the rules on power, some like them higher, some like them lower, they seem to be all over the place on usage!
On normal carto's, carto tanks etc., you take the ohm's and add 2 as a general easy rule. 1.7 ohms, add 2, 3.7v. 2.8 ohms, add 2 and 4.8v. And so on. This will keep you right around 8 watts, what most people like, but again, the Vivi's don't seem to fall under these standards for whatever reasons.