In a mixing bowl, combine flour and vegan butter until the mixture resembles coarse crumbs. Gradually add ice water until the dough comes together. Divide the dough in half, shape into discs, and refrigerate for 30 minutes.
Roll out one disc of dough on a floured surface and fit it into a pie dish. Trim the edges.
In another bowl, mix rhubarb, sugar, cornstarch, and vanilla extract. Pour the filling into the pie crust.
Roll out the second disc of dough and place it over the filling. Trim and crimp the edges to seal. Cut a few slits in the top crust to allow steam to escape.
Bake for 50 minutes or until the crust is golden and the filling is bubbly. Let cool before serving.