Put the vinegar in a small nonreactive pan over medium-low heat. Bring to a boil, then immediately lower the heat so it bubbles gently. Reduce at a low simmer to 1/4 cup, about 20 minutes; it should be thickened and syrupy. It will thicken a little more as it cools. Serve warm or let cool, then store in an airtight container indefinitely.