Have you checked out some costume patterns? Sometimes they have vests and things like you are looking for. I remember seeing one recently. I will have a look.http://simplicity.com/dv1_v4.cfm?design=9966
You could just sew the side panels together instead of having the lace up sides. http://simplicity.com/dv1_v4.cfm?design=4484
This isn't an underbust but it could be altered pretty easily I think if you wanted a closed front.
I am so sure that I saw a costume pattern with a vest like you want but I can't find the exact one sorry.