Preheat oven to 350°.
In a bowl, combine the first four 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