Preheat oven to 350*. Spray or butter a 9x13 inch glass pan. (Don't use metal because acids will react with pan).
Place rhubarb in pan, scatter strawberries over top.
In a medium sauce pan: Combine fruit juices and enough cold water to make 1 1/2 cups liquid. Add cornstarch and stir to combine. Do this when ingredients are cold to prevent lumps.
Add sugar to pan. Over medium heat, bring to a boil. Stir constantly to avoid lumps. Stir until mixture turns clear and starts to thicken, but still pourable. Remove from heat and stir in vanilla extract.
Slowly pour evenly over the fruit in your pan.
In separate bowl combine topping ingredients. Blend with a fork or use fingers, mix until topping is coarse and begins to hold together. Sprinkle over the top of fruit and filling mixture.
Bake 50-60 minutes. Topping will be crispy and golden brown. Filling will be bubbly.
VARIATION: Build in layers. Place half your topping mixture in bottom of pan first, add fruit and cornstarch / sugar mixture. Sprinkle remaining topping over all.