Combine sugar, 1/4 cup water, and lemon peel in a small saucepan. Bring to a boil; remove from heat and steep for 5 minutes. Remove peel and reserve. Refrigerate syrup until completely cool.
In a 2-quart pitcher, combine lemon syrup, limoncello, vodka, and lemon juice; stir and divide among six 8-ounce glasses. Garnish with a reserved lemon peel skewered on a toothpick.