Freeze half of the grapes. Puree the other half in a blender until smooth. Strain into a pitcher and stir in the simple syrup. Chill for at least 1 hour.
Divide the frozen grapes among 4 tall glasses. Stir the vodka into the pureed grape mixture and pour into the glasses, filling about three-quarters of the way. Top off with club soda. Serve.
The vodka in this cocktail turns the grape mixture slightly brown the longer it sits, so stir it into the pitcher just before serving.