Begin by slicing the rhubarb into small, even pieces and placing them in a mixing bowl. Toss the chopped rhubarb with the sugar and salt, allowing it to macerate for at least 30 minutes.

Transfer the macerated rhubarb to a saucepan over medium heat. Bring the mixture to a boil, then reduce the heat and let it simmer for about 5 to 10 minutes, stirring occasionally, until tender and thickened.

Once thickened, carefully transfer the cooked rhubarb filling to a bowl and stir in the vanilla extract. Allow the filling to cool to room temperature.

Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish with butter and line it with parchment paper.

In a large mixing bowl, combine the all-purpose flour, old-fashioned rolled oats, baking soda, salt, ground cinnamon, brown sugar, and granulated sugar. Mix well to create a cohesive base.

Add the cold, sliced unsalted butter to the dry ingredients. Work the butter into the mixture until it resembles small peas.

Press about two-thirds of the crumb mixture evenly into the bottom of your prepared baking pan. Bake for about 10 minutes, or until just set.

Spread the cooled rhubarb filling evenly over the base. Crumble the remaining dough over the top generously, and return the pan to the oven. Bake for an additional 20 to 25 minutes.

Allow the bars to cool completely in the pan on a wire rack. Once cooled, lift the bars out using the parchment overhang and cut them into squares.