In a large bowl, combine the rhubarb, sugar, and vanilla extract. Stir until combined evenly.
Dissolve the cornstarch by stirring into the lemon juice and pour over fruit mixture. Toss to combine.
Spread the fruit mixture evenly into a 9x13 baking dish.
Prepare the topping by combining flour, oats, brown sugar, cinnamon, and a pinch of salt in a medium mixing bowl. Cut cold butter into flour/sugar mixture using a pastry blender until the mixture clumps together into small crumbly pieces.
Spread the streusel topping over the rhubarb mixture in the baking dish.
Bake for about 45 minutes or until the filling is bubbly and the crisp topping is golden brown.