Wash the lemons thoroughly. Using a peeler, take off the skins being careful not to get any of the white lemon "pith" onto your peelings or it will add bitterness to your limoncello. Place peels into a large wide-mouth jar with all the vodka. Seal tightly. Place the jar in a cool, dry place for 2 to 3 weeks. Shake the jar to remix everything everyday or so. You will notice the color of the liquid changing to yellow as the peels lose their color.
After 1 week, dissolve the sugar completely with the water by heating it on the stove in a large saucepan. Let simple syrup cool to room temperature. Strain the lemon peels from the alcohol and discard. Mix the alcohol with the simple syrup. At this point you can serve it right away or pour the alcohol into a bottle with the simple syrup and place in the freezer to chill. Once chilled it becomes thick and syrupy.