Preheat broiler. Toss plums with Sugar, vanilla, and cinnamon in a 1 1/2 to 2 quart (9- to 10-inch round) gratin or other flameproof shallow dish (not glass). Let plums stand 5 minutes in dish to macerate. Add water to dish and broil plums 6 to 8 inches from heat, gently stirring once or twice, until fruit is softened, 7 to 10 minutes. Serve fruit and juice from pan topped with sorbet.