shudder Only if he’s planning on serving the wedding breakfast.
Vests are totally appropriate! I don’t understand why anybody would say otherwise.
A man is almost always safe in a suit. I think a vest is a lovely addition. (And I’ve seen quite a few of them on the red carpet lately if whatever Chris Pine wears enters into your calculations.)
What’s the good of being gay if you can’t wear a vest on such an occasion?
The vest would be fine. Don’t wear jeans. You need some kind of suit jacket, sport coat, or blazer if you don’t wear the vest. Don’t go so far as a tuxedo. Really, anything between a tuxedo and a nice suit and tie with slacks will be fine.