Preheat your oven to 425°F (220°C).
In a large mixing bowl, combine the chopped rhubarb, granulated sugar, cornstarch, salt, vanilla extract, and lemon juice. Stir until the rhubarb is evenly coated.
Roll out the prepared pie crust and fit it into the 9-inch pie dish. Trim any excess overhang.
Pour the rhubarb mixture into the pie crust, spreading it evenly.
Dot the filling with small pieces of butter on top.
Roll out the second pie crust (if using homemade) and place it over the filling. Seal the edges by crimping with your fingers or a fork.
Cut slits in the top crust to allow steam to escape.
Optional: Brush the top crust with a beaten egg for a golden finish.
Place a piece of aluminum foil around the edges of the pie to prevent over-browning.
Bake in the preheated oven for 25 minutes, then remove the foil and bake for an additional 15-20 minutes, or until the crust is golden and the filling is bubbly.
Allow the pie to cool for at least 2 hours before serving to set the filling.