Coarsely crush the cardamom seeds, split the vanilla pod lengthwise, scrape out the seeds and put them in a saucepan with the cardamom and other spices. Pour in the wine and lemon juice. Bring to the boil then turn off the heat, cover and leave to infuse.
Rinse and mop the grapes. Put them in the saucepan when the liquid is lukewarm, add the grape juice. Let cool completely and refrigerate until ready to serve.
Wet the rim of four tall glasses or cups and invert them in a saucer containing the powdered sugar, to frost the rims.
Place the grapes in the glasses (without touching the edges). Filter the cooking juice and pour it over the fruit.
Serve chilled.
