Using lemon slice, moisten the rim of a chilled cocktail glass; set lemon aside. Sprinkle sugar on a plate; hold glass upside down and dip rim into sugar. Discard remaining sugar on plate. Fill a shaker three-fourths full with ice. Add vodka, limoncello and lemon juice; cover and shake until condensation forms on outside of shaker, 10-15 seconds. Strain into prepared glass. Garnish with lemon slice.