Heads up on using a 901 on a Vapor King battery. First, the auto batteries don't work so well with the 901 atty, it is just hard to trigger the battery. Second, on the manual batteries, the draw is still pretty tight with the 901 whistle tip in place, but the draw is good if you take the whistle tip off. But then, so you don't burn your lips you need to cut the end off of a carto condom and slip it over the open 901 for a nice comfy place to rest your lips while you direct drip the 901.
Of course, I'd just recommend direct dripping a KR instead, but flavor changing while dripping is easier with the 901.