Preheat oven to 350°. In a large bowl, combine the first 4 ingredients. Cut in butter until crumbly. Press half into a greased 2-1/2-qt. baking dish or a 9-in. square baking pan. Cover with apples.
In a small saucepan, combine the sugar, cornstarch, water and vanilla. Bring to a boil; cook and stir 2 minutes or until thick and clear. Pour over apples. Sprinkle with remaining crumb mixture.
Bake 60-65 minutes or until apples are tender. Serve warm, with ice cream if desired.