Wash the lemons with a vegetable brush and hot water to remove any reside of pesticides or wax. Carefully zest the lemons with a zester or vegetable peeler so there is no white pith on the peel. -- In a 1-gallon glass jar, add one fifth vodka and the lemon zest. Cover the jar and let sit at room temperature for at least 10 days, and up to 40 days in a cool dark place. -- In a large saucepan, combine the sugar and water and cook until thickened, appx. 5 to 7 minutes. Let the syrup cool before adding it to the limoncello mixture. Add the additional fifth of vodka. Allow to rest for another 10 to 40 days. -- After the rest period, strain and bottle, discarding the lemon zest. Keep in the freezer until ready to serve.