Preheat your oven to 375°F (190°C).
In a mixing bowl, combine flour and butter. Mix until it resembles coarse crumbs.
Add ice water gradually until the dough comes together. Divide into two discs, wrap in plastic, and chill for 30 minutes.
Roll out one disc and fit it into a pie dish. Set aside.
In another bowl, mix rhubarb, blueberries, sugar, cornstarch, lemon zest, and vanilla extract.
Pour the filling into the pie crust. Roll out the second disc and place it over the filling. Seal and crimp the edges.
Cut slits in the top crust to allow steam to escape. Bake for 60 minutes or until golden brown.
Let the pie cool before serving.