Preheat oven to 375°F (190°C). Grease and flour a 9x13 inch baking pan.

In a large bowl, whisk together the flour, rolled oats, brown sugar, baking soda, and salt for the crust and topping mixture.

Pour the melted butter over the dry ingredients and stir until combined and crumbly. Reserve 1.5 cups of this mixture for the topping.

Press the remaining crumb mixture evenly into the bottom of the prepared baking pan to form the crust.

In a separate medium bowl, combine the chopped rhubarb, granulated sugar, cornstarch, and water for the filling. Stir until the rhubarb is evenly coated.

Pour the rhubarb filling evenly over the prepared crust. Sprinkle the reserved crumb mixture evenly over the rhubarb filling.

Bake for 35-40 minutes, or until the topping is golden brown and the rhubarb is tender and bubbly.

Let the bars cool completely on a wire rack before cutting into squares. Stir in the vanilla extract into the rhubarb filling just before pouring it over the crust.

Add vanilla extract to the rhubarb mixture. Stir to combine. Pour the rhubarb mixture evenly over the crust. Sprinkle the reserved crumb mixture evenly over the rhubarb filling.