Bring the sugar with the vanilla powder (you can also use the pulp of half a pod) and water in a saucepan to simmer, 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, it takes about 40 minutes until the sorbet is frozen.
You can also let the sorbet freeze in the frozen food, but then stir it very often.