It's a little hard to figure out the cause, but I'll take my best guess....
You say it's for MtF cosplay... So does that mean the vest is meant to accommodate breasts and you're lacking them? Have you tried padding your chest or wearing fake breasts under the vest?
What might be going on is that a lack of breasts causes the front to collapse so the back swings away from your body. Padding your chest may help the garment pull into place like it's supposed to. Try that first and see what it does.
If that does not help, then you will have to do a lot of work on the back of the vest to make it fit properly. It's slightly too wide for your back and it's not tailored properly to the back of your waist. You'll need to get a friend to help. Have them pin out the excess fabric until it fits right, and then you'll have to re-sew the seams in the back to remove the excess.
FF9 cosplay? Hit me up!
I now have a Tumblr
. Sometimes I use it.