Tip the vanilla into a bowl with the sugar. Bring a kettle of water to the boil and pour 250ml over the sugar. Stir until completely dissolved, then leave to cool. Meanwhile, peel the mangoes and cut away all the flesh, then blitz in a food processor until you have a smooth purée. Stir the purée into the syrup and fish out the vanilla pod. Freeze the purée as before or churn in an ice-cream machine to make a sorbet. Serve with sliced strawberries.