Place the pitted cherries, lemon juice, vanilla extract, water, and sweetener (if using) into your blender.
Blend on high until the mixture is smooth, vibrant in color, and the cherries are fully mashed—about 30 seconds.
Pour the cherry mixture into a shallow dish or baking sheet and place it in the freezer.
Let the mixture freeze for about 4 hours, stirring every 30 minutes with a fork to break up ice crystals and create a slushy consistency.
Once the mixture is firm but scoopable, scoop it into glasses.
Serve immediately, garnished with fresh cherries or a lemon wedge if desired, and enjoy the icy, tart flavor burst.