In a large mixing bowl, combine rhubarb, strawberries, sugar, cornstarch, lemon juice, and vanilla extract. Mix well.
Roll out one pie crust and fit it into a 9-inch pie dish. Pour the fruit mixture into the crust.
Roll out the second pie crust 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 45 minutes or until the crust is golden brown and the filling is bubbly. If the edges of the crust start to brown too quickly, cover them with foil.