Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking dish.
In a large bowl, combine the chopped rhubarb, granulated sugar, 1 tablespoon of flour, vanilla extract, and ground cinnamon. Mix well and spread the mixture evenly in the prepared baking dish.
In another bowl, mix together the rolled oats, 1 cup of flour, brown sugar, melted butter, baking powder, and salt until crumbly.
Sprinkle the oat mixture evenly over the rhubarb layer in the baking dish.
Bake in the preheated oven for 35-40 minutes, or until the top is golden brown and the rhubarb is bubbling.
Allow to cool for at least 10 minutes before serving.