Combine half the prepared cherries (16 ounces, or around 1 1/4 cups) and 1/4 cup dark brown sugar in a medium saucepan over medium-high heat. Cook, stirring frequently to prevent the fruit from scorching, for 15 minutes or until the cherries have released their juices and darkened in color. Remove from heat and stir in the remaining cherries, 1/2 cup granulated sugar, 1/4 cup cornstarch, and 2 teaspoons pure vanilla extract until well combined. Let cool to room temperature before filling the pie; the filling can be stored in an airtight refrigerator for up to 3 days.