I ripped it all out. This is the first time I have taken a finished project and frogged it. It may have been possible to cut the vest and sew it together to the correct size, but I've never done that and didn't have enough confidence that it would come out well. I do know that if I have the yarn back, I can knit a smaller vest. So I unraveled the ribbing around the neck and armholes, picked out the seams, and frogged the entire back.
When I hung it up to dry I tried using clothes hangers to weight down the skeins. It worked fairly well, but I think I would have needed more weight to take out all the kinks. It made for a strange set of wind chimes in our bathroom for a few days.
Yesterday the yarn was finally dry and I wound it into center-pull balls. Here it is with the yet-to-be-frogged front of the vest, and the first two rows of the second attempt. This time I have a vest of his that fits him well, so I'll be constantly comparing to that, instead of working from a sticky note of measurements.