Bring the sugar to the boil with the vanilla powder (you can also use the scraped pulp of half a pod) and water in a saucepan, the sugar must dissolve completely. Let the sugar water cool down.
Puree the melon pulp.
Stir in the sugar water, lemon juice and pour into the ice cream maker, takes about 40 minutes to freeze the sorbet. You can also freeze the sorbet in the freezer, but then stir very often.