Many leafy vegetables are naturally bitter and Turnip Greens are no exception. A touch of acid will help offset that bitterness. Getting that acid from the vinegar can also add a subtle bit of flavor in the background. In this recipe, I use Cane Vinegar (5% acidity) but you could use white or apple cider vinegar instead. Also, the cane syrup (or a substitute) as well as the long braising time also helps counter the bitterness and balance the smoky background.