Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan with butter or non-stick spray.

In a medium saucepan, combine the chopped rhubarb, granulated sugar, cornstarch, and water. Cook over medium heat, stirring constantly, until the mixture thickens and becomes clear. Remove from heat and stir in the vanilla extract.

In a large mixing bowl, whisk together the flour, rolled oats, brown sugar, baking soda, and salt until well combined.

Pour the melted butter over the dry ingredients and stir until the mixture is crumbly and the butter is evenly distributed.

Press half of the oatmeal mixture firmly into the bottom of the prepared baking pan to form the base crust.

Spread the cooked rhubarb filling evenly over the bottom crust layer.

Sprinkle the remaining oatmeal mixture over the top of the rhubarb filling, pressing down very lightly.

Bake for 35 to 40 minutes or until the top is golden brown. Allow the bars to cool completely in the pan before slicing into squares.