Place the blueberries and sugar in a mason jar. Muddle the blueberries a bit to begin releasing some of the juices. Cover it with a loose lid or cheesecloth and let sit out on your counter for about 3 days. 

At the end of three days it should be syrupy. Stir in the vinegar and let it sit out for 1 more day. Strain through a fine mesh strainer and store in a lidded jar in the fridge (it should keep for about 6 months.)