Preheat the oven to 425℉. Line a 9-inch pie plate with one pie crust.
In a large bowl, whisk together the sugar, cornstarch and orange zest. Stir in the rhubarb, strawberries. Drizzle the vanilla extract over the coated fruit. Spoon the fruit on top of the pie crust in the plate. Dot the top of the fruit with the little butter pieces.
Place the 2nd pastry on top. Cut slits to allow escaping steam. Press the edges around the plate to seal. Brush the pie with egg wash, and sprinkle a little bit of sugar over the top.
Cover the edge of the pie with a pie shield, or arrange a 2-inch strip of foil around the edge (prevents from browning too much).
Cover the edge of the pie with a pie shield, or arrange a 2-inch strip of foil around the edge (prevents from browning too much).