Coarsely crush the cardamom seeds. Split the vanilla pod lengthwise, scrape out the seeds, and add everything to a saucepan with the cloves, star anise, and cinnamon stick. Pour in the Floc de Gascogne and lemon juice. Bring to a boil, then remove from heat, cover, and let infuse.
Rinse and pat dry the grapes. Add them to the saucepan once the liquid is lukewarm, then stir in the grape juice. Cool completely and refrigerate until serving.
Moisten the rims of four tall glasses or cups and dip in powdered sugar for a frosted rim.
Divide the grapes into glasses (avoiding the rims). Strain the infused liquid and pour over the fruit.
Serve well chilled.
