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.
Plums can be broiled 2 hours ahead and divided among serving dishes while still warm.