Grape sugar tempers the acidity of balsamic vinegar, and the aging in wood gives it interest. Adding sugar to vinegar is not the way to get that push-pull, although virtually every producer of balsamic vinegar outside Modena does that, and quite a few in Modena as well. Balsamic vinegar starts not with wine, in which most or all of the grape sugar has fermented to alcohol, but with the cooked-down juice of very sweet grapes, itself quite thick. Producers who want to save time and money and satisfy customer demand add caramel or cooked-down grape juice to already fermented vinegar. Although the deep purple-brown color and heavy density may be right, the flavor is somewhere between Passover wine and grape jelly.
Most fruit vinegars are a sham too.
I love cider vinegar on fresh melon and over fruit salad, and recently looked hard for a source of wood-aged cider vinegar, which seems to have fallen out of fashion. Recently I discovered the unusual and worthy maple vinegar, which is fermented from maple syrup rather than simply flavored with it. Sherry vinegar is as interesting as balsamic but far less well known. Sherry passes through many barrels and is continually mixed with wines of various ages, so the end product, like balsamic vinegar, contains both relatively young and very old liquid. Although sherry vinegar is never as syrupy as balsamic, and tastes more potently acidic, it often has comparable balance and depth.