In a saucepan, combine the berries, sugar, lemon juice, and vanilla.
Simmer over medium low heat, about 5-10 minutes, or until warm and juicy.
Remove 1/2 cup of the juice and stir in the cornstarch. Add back to the pot, and cook an additional 5 minutes or until thickened.
Turn off the heat, and add in the butter. Allow to cool for 15-20 minutes.
Preheat the oven to 400 degrees.
Line a pie tin with one crust, and add in the filling. Top with the second crust, either in a lattice form, or full. (If full, cut slits for venting).
Brush with egg wash. Sprinkle with sugar.
Bake for 45 minutes, or until golden brown and bubbling. If needed, add tin foil to the top if the crust is browning too quickly.
Allow to cool before slicing.
