Mix rhubarb, sugar, cornstarch, water and vanilla together.
Cook in a saucepan until thick, then set aside.
Mix topping ingredients until crumbly.
Press ¾ of the topping mixture into a 9 × 13-inch pan.
Pour rhubarb mixture over the crust.
Sprinkle the remaining topping mixture over the rhubarb.
Bake at 350 degrees for 30 minutes.