Balsamic vinegar is different from most other vinegars. Balsamic is made by cooking down and evaporating off liquids over time (at least 12 years) from grape must (unfermented juice) until it's the consistency of syrup. Concentrating the juice also concentrates the sugars.
You can cut the balsamic vinegar you use with a high quality red wine vinegar. You'll still get a good bit of the flavor and less of the carbs.