Great question. Not an exact comparison, but here's my experience with an ARC and Vapor. My buddy demo'd a 68" Arc S this summer, and I strapped my bindings on it for a weekend. I'm 175lb, so I think it was a pinch big for me, but what a great ski. I clearly see why they named it the "ARC." It offered the most predictable arcing turns I've ever experienced on a ski. If my form was a little off, it still came around just like it was supposed to. I was very close to buying one shortly after trying it out, but a good deal came up on a 67" 2017 Pro Build. I had read great things about the Pro Build, and even read some comparisons between the Vapor and ARC.
I decided to pull the trigger on the Pro Build. I'm 3 weeks into the ski, and happy with the decision. The turns aren't as predictable, but the angle and space I'm able to get with the ski is rewarding. I'm still getting it dialed in, but look forward to where things are going.
I personally don't think you can go wrong with either ski. If your weakness is in the turns, I would recommend the ARC. If you have trouble creating space and angle coming out of the turns......I would go with the Vapor. I'm sure others could offer a lot more details, but that's my .02.