Pour the measured water and marsala or sherry into a saucepan, then add the sugar. Slit the vanilla pod lengthways and scrape out the black seeds from inside the pod. Add these to the water with the pod, then gently heat the mixture until the sugar has dissolved.
Place the peach halves in an overproof dish so that they sit together snugly. Pour over the hot syrup, then cover and cook in a preheated oven, 180°F (350°F), Gas Mark 4, for 20 minutes.
Scatter over the raspberries. Serve the fruit either warm or cold. Spoon into serving bowls and decorate with the vanilla pod cut into thin strips.