Combine vinegar, mustard, sugar, salt and pepper. Drizzle in the oil, whisking constantly, until thickened. Set aside. Cook the orzo according to package directions; drain, rinse under warm water and drain again. Toss with half of the vinaigrette. Stir in the bell peppers, onion, zucchini, olives and half the parsley. Spoon onto a large serving platter. Place the clams and oysters on a grill rack over high heat, 3 inches from the heat source. Grill the clams for 5 minutes or until they open. Grill the oysters for 5 minutes to 10 minutes, depending on size, or until they open.  To serve, place the opened clams and oysters atop the orzo salad. Drizzle with the remaining vinaigrette and sprinkle with the remaining 2 tablespoons chopped parsley. Serves 4.