Prep the fruit: Rinse and dry. Halve bigger grapes. If using frozen blueberries, don’t thaw.
Make the filling: Toss grapes and blueberries with sugar, cornstarch, lemon zest and juice, salt, and vanilla. It should look glossy and slightly sandy.
Line the pan: Fit the bottom crust into a 9-inch pie plate. Chill for 10 minutes.
Fill: Pour in the fruit. Dot with butter.
Top it: Add the top crust or do a quick lattice. Seal and crimp. Vent slits if using a full top crust. Brush with egg wash and sprinkle sugar.
Bake: 425°F (220°C) for 20 minutes, then reduce to 375°F (190°C) and bake 30-35 minutes more. You want vigorous bubbling in the center.
Cool: Let it rest at least 3 hours so the juices thicken. Yes, waiting hurts. Worth it.