Measure the vinegar into a small saucepan and make note of the height of the vinegar on the side of the pan. Bring the vinegar to a simmer. Continue simmering for at least 20 minutes, swirling frequently. Continue simmering and swirling till the vinegar has reduced to about half it’s original volume.
Do not boil the vinegar vigorously for long, as it may become candy. Take the pan off the heat and allow the vinegar to cool, then test the consistency. It should cling to the back of a spoon and drip off slowly. If it’s still too thin, return it to a simmering temperature and continue the reduction. It’s best to determine if the reduction is complete while the vinegar is cool, as it thickens as it cools.