Fill your sous vide cooker with water and preheat to 135F/57C.
Using a sharp kitchen knife or vegetable peeler, peel the lemons to remove the zest. Be careful not to peel too deep or you'll include the pith. You can remove the pith by scraping them gently.
Place the lemon zest into a large ziplock bag. Add the vodka. Cook it for 2 hours.
Place the sugar in a saucepan with water. Bring to a boil over medium heat. Stir once in a while to allow the sugar to dissolve completely.
Once the cooking time has passed, strain the mixture into a pitcher. Add the sugar mixture to it and stir to have all the ingredients incorporated well.
Pour the limoncello into bottles and chill in the freezer before serving.