Whisk together first 3 ingredients in a medium bowl. Place peach slices in a large bowl. Sprinkle sugar mixture over peaches, and toss until combined. Sprinkle vanilla over peach mixture, add raspberries, and toss gently. Let stand while preparing topping.
Whisk together brown sugar and 2 cups flour in a medium bowl. Cut in butter with a pastry blender until it resembles coarse meal. Stir in oats and walnuts. Pour peach mixture into a lightly greased 13- x 9-inch baking dish. Sprinkle with topping (dish will be full).
Bake at 350° for 45 minutes or until filling is bubbly and top is golden. Serve warm. Garnish, if desired.