Preheat oven to 350 degrees F. Spray a 9-inch square baking pan generously with baking spray.
Make the topping by mixing the flour, brown sugar, oats, butter, vanilla, and salt until crumbly, pea-size pieces form. Set aside.
In a large bowl, combine the grapes, blackberries, sugar, and cornstarch. Pour into the prepared pan. Sprinkle the crumb topping evenly over the fruit. Bake in the middle of the preheated oven for 40 to 45 minutes, until the top is golden brown and the fruit is bubbling in the center. Serve warm with whipped cream or ice cream.