1) Combine sugar, water and vanilla pod and seeds in a small saucepan and cook until the sugar is completely dissolved. Add the plums and cook until very soft. Transfer the mixture to a food processor and process until smooth.
2) Spoon a tbsp of the puree into each of 4 champagne flutes and fill to the top with prosecco.
3) Garnish each glass with a plum slice and a mint sprig.