Preheat the oven to 425°F (220°C).
In a large bowl, combine strawberries, rhubarb, sugar, flour, lemon juice, and vanilla.
Let the mixture sit for about 15 minutes to allow the juices to develop.
Roll out the pie crust and place it into a 9-inch pie pan.
Pour the strawberry-rhubarb mixture into the crust, spreading it evenly.
Dot the filling with small pieces of butter.
Roll out the second pie crust and cut it into strips for a lattice top.
Weave the strips over the filling, sealing the edges.
Brush the crust with beaten egg for a golden finish.
Bake for 45-50 minutes or until the filling is bubbly and the crust is golden brown.