Preheat oven to 350 degrees. Mix first 5 ingredients and press ½ of the mixture into a 9 x9-inch pan that has been sprayed with cooking spray. Top with rhubarb.
Combine the sugar, corn starch, and water in a saucepan and cook over medium to medium-high heat, stirring constantly until slightly thickened, about 3-5 minutes. Remove from heat. Add the vanilla. Pour the mixture over the rhubarb.
Top with the remaining crumb mixture and bake for 50 minutes to an hour until lightly browned.