Instructions: Scrub and rinse the oysters and set aside. Light the charcoal grill and allow 30 to 45 minutes for it to reach the proper temperature. Reduce the vinegar, Vermouth, and shallots in a saucepan over moderately high heat until only 2 tablespoons of liquid remain. Adjust to very low heat. Cut the butter into 1/2 inch slices and whisk one piece at a time into the reduced liquid; whisk constantly with each addition until the sauce is creamy and thick. Add the lemon zest and season to taste. Keep the sauce warm over water that is the same temperature as the sauce. Line two large shallow pans with rock salt and heat in the oven. Put the oysters, flat-side-up, directly on the grill when the coals are pure white hot.

Remove with tongs when the shells begin to open. Wearing thick canvas gloves, shuck the oysters, discard the top shells, and place the oysters in their deep shells in the hot rock salt to keep warm. Serve the oysters with the sauce and lemon wedges.

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