Add cherries sugar, vanilla, and lemon juice. Cook on medium heat for 5 minutes. Genly smash some of the cherries and continue to cook for 10 minutes.
Make a cornstarch slurry with cornstarch and water in the meantime. Add the slurry to the cherry mixture, cook for a minute and remove from the flame.
Cool completely and transfer to a airtight container. Keep refrigerated until use.