The vest doesn't look boned, just really, really fitted to the point that there is absolutely no ease left in it. It's a good thing your pleather has stretch.
I'd start with a very plain shirt pattern that fits around the neck and shoulders, and just fit the shit out of it until it was snug. It will be a lot easier to fit it if you have a friend to help you out while your mockup is on you.
You could look specifically for stretch patterns (either 4-way stretch or 2-way stretch, depending on what your fabric is), it would make your life a lot easier. Kwik-Sew is usually the pattern company I go to for stretchwear. They have a few that are plain shirts.
And on an aside, do not work straight away with your pleather. Make your mockup out of something cheap and disposable, then make the real thing. I know, there's not much time left before Halloween, but it would suck way worse to fit with the real fabric, royally mess up, and then waste it all.
FF9 cosplay? Hit me up!
I now have a Tumblr
. Sometimes I use it.