Whisk flour and salt together in a bowl. Cut in shortening with 2 knives or pastry blender until the mixture resembles coarse crumbs. Add cold water, 1 tablespoon at a time, tossing with a fork, until flour mixture is moistened. Do not add more water than you need: when you squeeze a handful of the moistened pastry mixture, it should form a ball.

Divide the dough in half and shape into balls. Wrap in plastic wrap and refrigerate, at least 1 hour or up to 3 days.

Preheat oven to 375°F (190°C).

Mix cherries, sugar, and tapioca flour together in a saucepan; let sit until the sugar draws out the cherries' juices, about 10 minutes. Heat the cherry mixture over medium heat until the juices begin to thicken and run clear, about 5 minutes. Remove from heat; mix in butter and vanilla extract. Let sit until filling has cooled to lukewarm, 5 to 10 minutes.

Roll out half the pastry to fit a 9-inch pie plate. Place bottom crust in pie plate. Spoon filling into crust using a slotted spoon to drain excess syrup; dot with butter. Cut left over dough into strips with a sharp paring knife or pastry wheel. Lay strips on top of the pie, pressing to seal the edges.