Mattress stitch is my preferred seaming method because it is so invisible--but I'd never seamed reverse stockinette before, so out came my trusty Reader's Digest Knitter's Handbook by Montse Stanley. It turns out it's not much different from stockinette mattress stitch; you just have to make sure you always catch the bottom of the ridge on one side and the top of the ridge on the other. So as I was working, I was chanting "Bottom, top, bottom, top..."
I think it turned out pretty well...as you can see below, there is a little ridge where the seam is, but I am thinking re-blocking the piece once it's all seamed up will solve that.
I still have some ends to weave in and the perfect buttons to find, but it looks like a vest now! And, HUUUUUUGE sigh of relief, it fits. Now to tackle those sleeves.