As a fellow busty female who cosplays Hawkeye, I'd be happy to help you. (Sorry, boys, I'm going to hijack the thread for girl talk for a second!)
First off, do you have a chest binder or compression shirt? With a compression garment, you can smooth your curvy shape so the vest fits smoothly over your chest. For example, this is my natural figure (as Lara Croft):
View attachment 346687
And this is me as Hawkeye, from approximately the same angle:
View attachment 346686
I use a binder from
T-Kingdom, and I highly recommend their products. However, if you don't want to spend that much or order internationally, you can use a medical compression shirt, waist cincher or elastic belly band to do more or less the same thing. (DO NOT use an Ace bandage or anything non-elastic to wrap your chest, as you can break ribs or damage cartilage by compressing too tightly.)
Since you'll be crossplaying, I'd suggest looking up some makeup tutorials on how to make your face more masculine (
here's a good one) and, if your hair isn't really short, invest in a good male hair-replacement wig (the one I'm wearing cost $35 on eBay, and I can get all three feet of my hair up under it). If you need a tutorial on putting long hair up under a wig, I have a step-by-step guide
here.
This thread should help you a lot with the actual costume assembly and props acquisition, but if you have any other questions about crossplaying, feel free to ask me; I've done about 50 male characters. Good luck!