Preheat oven to 350°. In a bowl, combine the first 4 ingredients; cut in butter until crumbly. Press half into an ungreased 9-in. square baking pan. Combine rhubarb and strawberries; spoon over crust.
In a saucepan, combine sugar and cornstarch. Stir in the water and vanilla; bring to a boil over medium heat. Cook and stir for 2 minutes. Pour over fruit. Sprinkle with remaining crumb mixture. Bake for 1 hour. Serve with ice cream if desired.