/cgl/ - Cosplay & EGL

How do I achieve a nice, smooth shape for a vest like in pic related ? I'm remaking a cosplay and my previous vest was a terrible and shapeless mess. I assume it was a mix of me being a girl crossplaying, using a fabric that maybe wasn't heavy enough, and probably the pattern not being correctly fitted. The main problem was sagging, with diagonal creases forming from the middle and up towards the armpits.

As I'm planning to remake it I've practiced padding my waist to get a flatter body shape, and I'm gonna make a mock-up, but my main question is : do I need something to weigh my fabric down and help it hold its shape ? Can I use some thick iron-on interfacing ? I've read about bespoke men clothes and padstitching, but I'll be using velvet and it will be hard to make discreet stitches.