Wash the lemons well with a vegetable brush and hot water; pat dry. Using a vegetable peeler, remove the peel from the lemon in long, wide strips. Scrape away any of the bitter white pith from the lemon peel with a paring knife.
Combine the lemon peels and one of the bottles of vodka in a large (at least 4 quarts) glass jar with a lid. Cover the jar and store it at room temperature in a dark cabinet or cupboard for 40 days. As the vodka sits, it will slowly take on the bright yellow color of the lemon zest.
In a medium saucepan, combine the sugar with 5 cups of water. Bring to a boil over high heat and let the syrup boil for 5 minutes. Let the syrup cool, then add it to the limoncello mixture, along with the remaining bottle of vodka. Cover the jar and return it to the cupboard for another 40 days.
Strain the limoncello into bottles and discard the lemon zest.
From Fine Cooking