Wash the cherries, remove the stem, place in a bowl and cover with water and the juice of the 3 lemons. Set aside for 2 hours and then rinse very well with plenty of water. Remove their pits and place in a large pot, also adding the sugar. Bring the pot to a boil with plenty of water and without stirring. Once boiled lower the heat and skim the foam from the top. Let the mixture simmer for about 15 minutes and let cool completely for at least 7-8 hours. Next, boil the sweet again for about 10 minutes over low heat. When the syrup thickens, add the vanilla and the juice of one lemon.