The main difference is age and sugar content. Red Wine Vinegar is fermented red wine that has turned sour; it is thin, sharp, and punchy. Balsamic Vinegar is made from unfermented grape juice (must) that is aged in wooden barrels; it is thick, dark, and significantly sweeter. The two really aren’t swap-able the way you can substitute a Rice Wine Vinegar for a White Wine Vinegar, primarily due to the sweetness difference. Replacing Red Wine Vinegar will add sweetness and a darker color to your dish and when you do the opposite you’ll need to add a teaspoon of honey or sugar to bolster the lower sweetness.