Zest 1 lemon into large pieces using a knife. Place lemon zest in a saucepan; top with 1 cup water and sugar. Bring lemon zest mixture to a boil, stirring occasionally, until sugar dissolves and liquid is syrupy, 5 to 10 minutes. Remove saucepan from heat and cool; remove and discard lemon zest.
Stir lemon juice and vodka together in a pitcher; pour in lemon syrup. Stir in enough water to fill the pitcher.
Fill glasses with ice; pour hard lemonade into each glass. Garnish each glass with a lemon slice.
