Preheat oven to 375°.
Combine first three ingredients. Scrape seeds from vanilla bean; add seeds to plum mixture. Discard bean. Toss mixture gently to combine. Spoon into a 13 x 9-inch baking dish coated with cooking spray.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and next 5 ingredients (through nutmeg) in a medium bowl; cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Sprinkle flour mixture evenly over plum mixture. Bake at 375° for 45 minutes or until plum mixture is bubbly and topping is lightly browned. Serve warm or at room temperature.