Preheat oven to 375°F (190°C).

In a large bowl, combine flour, brown sugar, oatmeal, and melted butter.

Mix until the ingredients form a crumble.

Press 3/4 of the crumble mixture into the bottom of a 9 x 9-inch baking pan.

In a separate bowl, combine sliced rhubarb, sugar, cold water, cornstarch, and vanilla.

Stir until the ingredients are well combined.

Pour the rhubarb mixture over the pressed crumble base.

Sprinkle the remaining crumble mixture evenly over the rhubarb filling.

Bake for 35 minutes, or until the topping is golden brown and the filling is bubbly.

Let cool slightly before serving.