I am still using a hyperflexx 4/3, a little warm but since I don't have a 3/2, it gets me by. I get the same chaffing really bad, all you need to do is coat your neck with body glide, takes care of the chaffing no problem.
honestly put a piece of duct tape over the seam that is bothering you. I wouldn't use Vasoline or anything that could eat the rubber. You could also try putting a piece of duct tape on your NECK which would probably work better.