Dark purple plums hold up better for baking than the lighter ones.
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.