Preheat oven to 350 degrees F. Place blueberries in piecrust. Set aside.
In a medium bowl, beat butter, buttermilk, eggs, and vanilla at medium-low speed with an electric mixer until butter is in nickel-sized pieces, approximately 1 minute.
In a small bowl, stir together sugar, flour, and salt; stir into egg mixture. Pour into piecrust.
Bake until set, 60 to 65 minutes. (Cover with aluminum foil to prevent excess browning, if necessary.) Let cool before serving. Top slices with whipped cream, if desired.