Preheat oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.

Prepare the crust/crumble: In a large bowl, whisk together the flour, granulated sugar, and salt. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs. Divide the mixture into two portions: 2/3 for the base, and 1/3 for the topping. Mix the vanilla extract only into the 2/3 base portion.

Press the 2/3 base portion firmly and evenly into the bottom of the prepared pan to form the crust.

Prepare the filling: In a separate bowl, gently toss the chopped rhubarb with the brown sugar, cornstarch, and lemon zest until the rhubarb is evenly coated.

Spread the rhubarb filling evenly over the prepared crust layer. Sprinkle the remaining 1/3 crumble mixture (without vanilla) evenly over the top of the rhubarb filling.

Bake for 40 to 45 minutes, or until the topping is golden brown and the filling is bubbly. If the topping browns too quickly, loosely tent the pan with foil.

Let the bars cool completely in the pan on a wire rack (at least 2 hours) before lifting out by the parchment overhang and cutting into 12 equal bars. Cooling is crucial for clean slicing.