Carefully remove the zest from the lemons using a microplane or rasp grater, being careful not to remove any of the white pith. Reserve the lemons to make lemonade or squeeze and freeze the juice for later use.
Put the lemon zest into a lidded glass container that is at least 2 quarts in size. Pour the vodka over the zest, and let sit in a cool, dark place for 7 days.
After 7 days, strain the liquid through a fine-mesh strainer lined with muslin or several layers of cheesecloth and return the vodka to the container.
Combine the water and sugar in a small saucepan set over high heat and cook, stirring continuously, until the sugar is completely dissolved, approximately 3 minutes. Once completely cool, add the sugar syrup to the vodka, and stir to combine.
Cover and put the container in the freezer for at least 4 hours. Limoncello can be stored in the freezer for up to 1 year.