What do I wear to this wedding?

The invitation says to "wear your best vintage attire".

I figured I would just find a nice dress. The problem is, I have a belly. I carry all of my weight in my stomach and look about 7 months pregnant (I'm not pregnant). I have been TTC for 2.5 years (including a MC), and really don't want people at the wedding assuming I'm pregnant. I need a dress that will hide my stomach. What kind of style should I be looking for?

The wedding will also be during the summer. I have been working on losing weight, but I don't think I will be much smaller by the time of the wedding.