Greek yogurt can be found at Supermarché PA. Dunno about the Fort St. store, but on Park Ave. it's with the other house-made Greek specialties (tzatziki, taramosalata, dolmades, etc.) next to the cheese section, not with the regular yogurts.
Just about any store with gourmet pretensions will have Champagne vinegar. Gourmet Laurier and ExoFruits, for example. Have even seen it in chain supermarkets on occassion. Latina and Olives et Épices might also be worth checking out.