Dice 1 Kg. melon, place in a casserole and cover with cold water; bring to a boil and let simmer for ten minutes.
Puree melon in a blender with its cooking water and work in 700 gr. sugar.
Mix in grated lemon peel and 1 Tbsp. vanilla. Let cook for 1 and ½ hour. Pour hot marmalade into small sterile glass-jars; hermetically seal and keep upside-down for 10 hours. Store in pantry.