Place plum halves, cut sides down, in an 8-inch square baking dish. Stir together 1/3 cup water and vanilla; pour over fruit.
Sprinkle 1/4 cup brown sugar evenly over fruit.
Bake plums at 450 for 25 to 30 minutes or until skins just start to blister. Divide fruit and syrup evenly among four bowls. Top each with 1 Tbsp. low-fat sour cream and 1/2 Tbsp. brown sugar.