Pack the plum slices into a pint-size glass jar or other heatproof container of a similar size. In a small saucepan, combine the vinegar, sugar, salt, star anise, and black peppercorns with ¾ cup water and place over medium-high heat. Bring the mixture to a boil, then pour it over the plums so that they are completely submerged. Let cool for 20 minutes on the counter, then cover and refrigerate for at least 4 hours or overnight. The pickles will become stronger the longer they’re allowed to sit in the brine. Store in the refrigerator for up to 4 weeks.