Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the Marsala or sherry, the water and sugar in a saucepan and bring to the boil.
Put the peach or nectarine halves and vanilla pod in the slow cooker pot and pour in the hot syrup. Cover with the lid and cook on low for 1–1½ hours or until hot and tender.
Lift the fruit out of the slow cooker pot and transfer to a serving dish. Remove the vanilla pod, then scrape the black seeds from the pod with a small sharp knife and stir the seeds back into the cooking syrup. Mix the cornflour to a smooth paste with a little cold water, then stir into the cooking syrup and cook on high for 15 minutes, stirring occasionally.
Pour the thickened syrup over the fruit, sprinkle with the raspberries and serve warm or chilled with spoonfuls of crème fraîche or vanilla ice cream, if liked.