Preheat oven to 350 degrees.
Spray 9 x 13 inch baking dish with cooking spray.
In a small saucepan, combine sugar, cornstarch and water and bring to a boil. Stir until thickened. Remove from heat and add vanilla.
In a medium mixing bowl, combine brown sugar, flour, oatmeal and butter. Mix well. Divide the mixture in half and place one half in the bottom of the baking dish and press evenly to form a crust.
Place sliced peaches over the crust. Pour sauce evenly over the peaches. Sprinkle remaining crumble mixture over the top.