Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large bowl, combine the chopped rhubarb, granulated sugar, and vanilla extract. Stir until the rhubarb is well coated. Spread the mixture evenly in the bottom of the prepared baking dish.
Sprinkle the dry yellow cake mix evenly over the rhubarb mixture. Do not stir.
In a separate bowl, mix the melted butter, water, and ground cinnamon together. Pour this mixture evenly over the cake mix layer. Again, do not stir.
Bake in the preheated oven for 45-50 minutes, or until the top is golden brown and the rhubarb is bubbly. Let it cool for about 10 minutes before serving.