I was wondering about the size of the bulb. Thought I read in one early report that the VX had turtled after broaching, which I think is basically impossible in the Viper?
We turtled the VX last November at Lake Murray when we got hit with a small microburst barely a couple hundred feet off the dock. Not a broach, just a two-sail knockdown in bizarre conditions that left two crew standing on the tank, the sails firmly in the water and one person perched on the keel. She went over very slowly at that point, but did go over. Since then I've seen plenty of broaches, but no more turtles so far. I'm sure there will be some eventually, and I'm sure the Viper is more resistant to turtling than the VX, but it's by no means impossible because I've personally seen carbon-mast/heavy keel Vipers do it. But it's very rare and takes the right combination of wind, waves and current to pull it off. Plus someone standing on the down-side tank. Nevertheless, either model is very easy to right if things do get that bad and they come up dry and ready to carry on.
Both are about the same amount of wet upwind, and both are easy to drive downwind. Both generally self-rescue if you broach. Upwind, the Viper drives like a keelboat and the VX like a dinghy. Downwind the VX is carrying less crew weight, less boat weight, less rocker and less sail area so the loads are much lower. Again, more dinghy-like than the Viper. I sail and race both my Viper and my VX with my two pre-teen kids and they love both, but the VX is easier for us. The personalities of the two boats will appeal differently to different people - you should pick the one that matches yours.