In a large bowl, combine the flour, oats, 1/2 cup sugar and cinnamon; cut in butter until mixture resembles coarse crumbs. Set aside half for topping.
Press remaining crumb mixture into an ungreased 11x7-in. baking dish. Top with rhubarb and berries.
In a small saucepan, combine cornstarch and remaining sugar. Gradually stir in water; bring to a boil. Reduce heat; cook and stir for 1-2 minutes or until thickened. Remove from the heat; stir in vanilla.
Pour over the fruit. Sprinkle with the reserved crumb mixture. Bake at 350° for 50-55 minutes or until golden brown. Serve warm or cold.
If using frozen rhubarb, measure rhubarb while still frozen, then thaw completely. Drain in a colander, but do not press liquid out.