I understand aregard’s issue because I have it, too. I ride dressage, so no half-seat unless we’re out galloping. Yes, the zipper end does contact/bump the pommel on the saddles with some vests. I try to buy shorter vests to minimize the problem, but lately, longer vests seem to be the norm. Most are cut longer in back, which is fine. The problem is with a zipper that extends too far down. If the vest has a double zipper – one that you can unzip from the bottom a little, that helps. Shorter in front, longer in back, all you vest designers!
Could be a conformational thing for individuals, too. If you have a long torso, maybe it’s not a problem.