Preheat the oven to 375°F (190°C). In a mixing bowl, combine chopped rhubarb with 1/4 cup of granulated sugar, 2 tablespoons of flour, and vanilla extract. Stir until the rhubarb is evenly coated. Pour the mixture into a greased 9x13 inch baking dish, spreading it into an even layer.
In a separate bowl, prepare the topping by mixing oats, flour, brown sugar, and melted butter. Use a pastry cutter or fingers to combine until the mixture resembles coarse crumbs and no large chunks of butter remain. Spread the topping evenly over the rhubarb filling.
Place the baking dish in the preheated oven. Bake uncovered for about 40 minutes, or until the topping is golden brown and bubbling around the edges. Listen for a gentle bubbling sound indicating it's ready, and smell the toasty aroma when the topping turns golden.
Remove the dish from the oven and let it cool slightly on a wire rack. The filling should be thickened and shiny, with a crispy, golden topping. Serve warm, optionally with a scoop of vanilla ice cream or whipped cream.
Optional: Dust the top with powdered sugar or garnish with fresh mint leaves for presentation. Serve directly from the dish, ensuring each portion gets plenty of the crispy topping and tender filling.
