Blend sour cream, sweetener, and vanilla together in a medium bowl. Add grapes, then sprinkle in mint while stirring until it is fully incorporated. Chill for at least 2 hours or place in freezer for about 1 hour before serving. Garnish with mint sprig.
* If you use very sweet grapes as I did, you will need just about a tablespoon of sweetener. Super-cold food needs more sweetener than room temperature food, so keep that in mind when you sweeten the sour cream or yogurt.