Yeah, you'll need to do a lot of work in the back to make it fit you. It sounds like you might just have a very small back waist. Sometimes this is called a "protruding seat" - in other words, your butt sticks out. =P I can sympathize, because I have that problem too. I always have to take things in at the back of the waist because my posture is terrible.
When you have someone to help fit it for you, make sure you're wearing any chest padding you plan to wear with the costume later. The chest padding does change how the vest fits you.
FF9 cosplay? Hit me up!
I now have a Tumblr
. Sometimes I use it.