Armhole alterations can be tricky even on vests. Playing with shoulder seams will alter both the armhole AND the neckline, areas which show ill-fitting issues more than others. That's one of the reasons I'd stay away from replacing the back. It's already fitted together. To make it balanced, you'd have to add to the front too, which you can't. A side seam addition won't change the neckline but will add width mostly where it's needed, at the chest/abdomen area.