Special equipment: 4 cocktail picks or 4-inch skewers; cocktail shaker
Thread 3 grapes onto each cocktail pick and freeze until ready to serve, at least 2 hours.
In a shallow bowl, mix together the lemon zest and coarse sugar.
Moisten the rims of 4 martini glasses with the lemon. Dip the rims of the glasses into the lemon-sugar.
Squeeze the juice from the lemon into a pitcher and add the grape juice and vodka. Fill a cocktail shaker with ice. Pour 1/2 of the vodka mixture into the cocktail shaker and shake for 20 seconds. Pour into 2 of the prepared glasses. Repeat with the remaining vodka mixture. Place a cocktail pick into each glass and serve.