Preheat the oven to 375°F.
In a medium mixing bowl, combine the chopped rhubarb, granulated sugar, all-purpose flour, and vanilla extract if using.
Stir until the rhubarb is evenly coated.
Pour the rhubarb mixture into a greased 8x8-inch baking dish and spread it evenly.
In a separate bowl, combine the oats, brown sugar, flour, cinnamon, and salt.
Add the cold butter cubes to the dry mixture.
Use your fingers or a pastry cutter to work the butter in until the mixture forms coarse crumbs.
Sprinkle the topping evenly over the rhubarb mixture.
Bake for 35 to 40 minutes, or until the topping is golden brown and the filling is bubbling at the edges.
Let it cool for 15 to 20 minutes before serving.