If your vinegar was in the refrigerator and got a bit thick, leave it on the counter to warm up. Don’t heat your vinegar on the stove unless you plan to use it right away. Heating the vinegar will actually increase or activate the bacteria in the vinegar, potentially leading to more solids. It will also change the taste and texture. That’s fine if you want a nice balsamic glaze, but not if you just want thinner vinegar.