Blend all but one grape with the lemon juice. Strain through a fine-mesh sieve. Measure 2 parts juice to 1 part vodka into a small pitcher or measuring cup. Divide the juice and vodka mixture evenly between the glasses and top off with a splash of champagne. Serve immediately.

Optional Sugar-Rimmed Glass: Spread the sugar onto a small plate. Slice the reserved grape in half and use it to moisten the rims of two martini glasses. Dip the rims in the sugar and shake off any excess. Fill glass with drink mixture and enjoy.