Preheat your oven to 375°F (190°C) and lightly grease a 9x13 inch baking dish.

In a large mixing bowl, toss the chopped rhubarb with the granulated sugar, 2 tablespoons of flour, and vanilla extract until the fruit is evenly coated.

Spread the rhubarb mixture evenly into the bottom of the prepared baking dish.

In a separate medium bowl, combine the 1 cup of flour, rolled oats, brown sugar, and ground cinnamon.

Pour the melted butter over the oat mixture and stir with a fork until the mixture is crumbly and well combined.

Sprinkle the crumble topping evenly over the rhubarb filling in the baking dish.

Bake for 40 to 45 minutes, or until the rhubarb is bubbling and the topping is golden brown.

Remove from the oven and let cool for 10-15 minutes before serving to allow the juices to set.