Just a waistcoat?

[Wareyin]

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.

[Good Egg]

As mentioned by others I think you should check with your hosts.

With regard to a waistcoat, some that are designed to be worn with trousers are too long to be worn with a kilt and just don't look right. You might look smarter in shirtsleeves with a tie and belt.

I would always want to encourage people to wear kilts but it must be appropiate to the occasion. At a wedding I think you really must conform to the level of dress requested in the invitation.

[Toxpert]

Adding to the “never look better than the bride”...
Never draw more attention than the bride.

If you are the only ‘kilted’ attendee... I would have serious reservations about wearing a kilt and kit.