In a bowl, mix cherries, sugar, tapioca, salt and vanilla; let stand for 15 minutes, stirring occasionally.
Lightly mist 6 cups of a 12-cup muffin tin with cooking spray, alternating to keep space between them. Unroll 1 pie crust onto a lightly floured surface. Roll out to about an 11-inch round. Use a 5-inch round cutter to cut out 6 rounds. Transfer rounds to muffin tin. Gently fit dough into muffin cups, leaving some overhang. Chill for 15 minutes. Repeat rolling with second pie crust, cutting out 6 3-inch rounds. Place on a parchment-lined baking sheet and chill.
Preheat oven to 375°F; line a baking sheet with foil. Stir filling; spoon about 2 Tbsp. into each dough-lined cup. Remove rounds from fridge and cut a small star out of center of each. Place one round on top of each pie. Fold edges of round over the overhanging dough; crimp to seal. Repeat with remaining dough rounds. Brush tops with milk and sprinkle with sugar, if desired. Place tin on baking sheet.
Bake for 30 to 35 minutes, until filling bubbles and crusts are golden brown (check frequently and cover with foil if crusts brown too quickly). Remove muffin tin to a rack to cool.