In a large bowl, stir frozen berries, vanilla, sugar, and flour until well coated. Pour into pie crust and even out the top.
In another separate bowl, measure out streusel ingredients. Using your hands, mix the streusel until it comes together and the butter is well incorporated throughout and everything is like moist sand. Spread 3/4 of the streusel over the frozen berries. {This recipe for streusel makes an obnoxious amount, especially for a pie, but that's how I like it. I used all but about 1/2 cup. Feel free to halve the topping recipe for a more socially acceptable amount of streusel for your pie.}
Bake pie for 1 hour or until crust and topping are golden brown. Cool completely before cutting and serving. Serve with whipped cream or vanilla ice cream.