I imagine that a vaporeon probably feels like slightly cool, soft, rubbery silicone and is very supple and squishy. They probably jiggle like jello when you gently spank them. :U
And just for the record, this one enjoys that a lot ;) She's playing it cool, but inside she's practically begging to feel her body jiggled and kneaded and massaged deeply like moist, living dough.