Whisk together the reserved cherry liquid, sugar, and cornstarch in a saucepan until smooth before turning on the heat.
Bring the mixture to a boil over medium-high heat while stirring constantly to prevent lumps from forming.
Boil the mixture for 3 to 4 minutes until it thickens and begins to bubble with a glossy texture.
Add the lemon juice and continue boiling for 1 additional minute while stirring constantly.
Add the frozen cherries all at once and stir gently to avoid breaking the fruit apart.
Return the filling to a light boil and cook until the cherries are heated through and coated in the thick sauce.
Remove the saucepan from the heat and stir in the vanilla extract.
Allow the filling to cool before using in pies or transferring to storage containers.
