Stuffed grape leaves are traditionally served with an egg-lemon sauce called avgolemono. Here's a recipe: Stuffed Grape Leaves in Egg-Lemon Sauce - Dolmathes Avgolemono
Also, by yogurt sauce, do you mean tzatziki? I think it would go great with your appetizers. I posted a recipe here some time ago.
Yes - that's what I had in mind. The egg/lemon sauce sounds great but I have some concern about spoilage. It looks like it all gets fully cooked - but it will have to be served outside in 100 degree weather and may sit there for an hour or more. Thoughts about this?