Amazingly, my plan has always been to run the vinyl all the way across the bathroom floor and underneath the shower pan, but I wasn't sure if that was the best idea. It's a nice feeling to know that my instincts were good, and I will incorporate your suggestion to go up the wall a bit with it.
However, your idea of creating a "drip pan" beneath the drain is fantastic and definitely something I will incorporate into my plans. Thanks for the all the great tips. I feel much more confident in going foward with this shower structure.
PS: Your previous suggestion about sandwiching pieces of sheet metal between the layers of 3/4 plywood and the floor joists I was sistering-up for strength WAS A GREAT ONE. Solid as a rock! Your clients are lucky to have such a clever contractor.