What you already have Vicar sounds entirely appropriate. Wear it with a clerical collar if you are attending as a chaplain or will be contributing to the event as a clergyman (eg saying the grace or giving a benediction). If you do not have a specifically clerical role at the event, replace the clerical collar and shirt with a white dress shirt and black bow tie.

A jabot or white cravat is also ideal but best worn with a doublet. I favor the Sheriffmuir which also works well with a clerical collar.

I am a Scottish Episcopalian priest but this should work for clergy of most denominations.

PS Garters according to the season is a novel idea but a bit subtle for their significance to be recognised by others.