Wash the mirabelle plums, remove the core and remove the bad spots. Then cut into small pieces. Cut through the vanilla pod and scrape out the pulp. Bring to the boil in a closed saucepan with the sugar, vanilla pulp, pod and apple juice. Cook on a low heat for 60 minutes. Then remove the vanilla pod and use a hand blender to grind into puree.
Pour hot, rinsed glasses and let cool on the head. The jam will keep for a few months if you store it in a cool, dark place.