I've attended one wedding kilted, with the bride's permission/insistence on wearing my kilt. It was an evening wedding here in Georgia, so even though I wore an Argyll Jacket, it soon came off. Even in my vest/waistcoat, I was better dressed than all but the wedding party.
I would ask the wedding party (of course no one will care but the bride), but would think your waistcoat will be perfectly acceptable.