Mix the rhubarb, strawberries, sugar, tapioca, flour, zest and juice of lemon, dash of cinnamon, and vanilla. Mix well in a large bowl and pour out into a chilled crust. Dot the top of the filling with the butter. Brush the edges of the pie crust with egg white wash. Roll out the other piece of dough and place over filling. Crimp to seal edges. Brush with egg white wash and garnish with large granule sugar. Collar with foil and bake at 425 degrees fahrenheit for 15 minutes. Decrease temperature to 375 degrees fahrenheit and bake for an additional 45 to 50 minutes, or until the filling starts bubbling. Higher altitude will take 450 degrees fahrenheit and 400 degrees fahrenheit respectively. Let cool before serving.