Using a vegetable peeler, remove the peel from the lemons in long strips (reserve the lemons for another use). Using a small sharp knife, trim away the white pith from the lemon peels; discard the pith. Place the lemon peels in a 2-quart pitcher. Pour the vodka over the peels and cover with plastic wrap. Steep the lemon peels in the vodka for 4 days at room temperature.
Stir the water and sugar in a large saucepan over medium heat until the sugar dissolves, about 5 minutes. Cool completely. Pour the sugar syrup over the vodka mixture. Cover and let stand at room temperature overnight. Strain the limoncello through a mesh strainer. Discard the peels. Transfer the limoncello to bottles. Seal the bottles and refrigerate until cold, at least 4 hours and up to 1 month.
Recipe courtesy of Giada De Laurentiis