Squeeze the fruit to make 1 liter of liquid. Cut open the vanilla stick and scrape it out. Add the vanilla pulp and stick with the preserving sugar to the liquid. Bring the whole thing to the boil, simmer for a few minutes. Remove the vanilla stick, if you want, you can cut it into pieces and put a piece in each glass.
Gelling test and filling. Turn the glasses upside down for a few minutes.