1.5 kg plums, ripe, but firm
750 g vinegar (use a fruity one)
250 g sugar (or more, if wanted)
1 vanilla bean, split lengthwise, seeds scraped out
3-4 bay leaves, fresh, optionally coarsely chopped
1 thumb-sized piece of ginger, fine slices
Combine vinegar, sugar and spices (if you want to remove them at the end, use a tea-ball, filter paper or the like). Bring to a boil and simmer for some minutes, till the sugar is dissolved.
Meanwhile wash and dry the plums, then pierce the skin all around with a little skewer or a paring knive point. Lay the plums next to each other evenly in the vinegar. They should not lie over each other. At moderate heat (liquid should not boil!), poach the plums for a few minutes, trying to make sure the skins don't burst. Get the plums out with a slotted spoon, drain well, and put into a jar. When all the plums are cooked, taste the liquid and add sugar or vinegar, if needed. Bring to a boil again and cook for 10 minutes to reduce.
Pour the vinegar reduction over the plums. Make sure they are completely covered: It's the vinegar that prevents spoiling. Therefore also make sure the jars are stuffed well and plums can't emerge from the liquid.
Seal jars and store in the refrigerator. They should keep for 3-4 months.