Place the oil in a Dutch oven or large heavy-bottomed pot and cook over medium heat. Add pancetta, if using, and cook, stirring often, until browned around the edges, about 5 minutes. Add onion, carrots, celery, garlic, and red pepper flakes and season with salt and pepper. Cook until the onion is translucent and carrots are tender, 10 to 12 minutes. Add the tomato paste, stirring to coat the vegetables, about 3 minutes. Add the whole tomatoes with the juices, crushing with your hands as you go and the parmesan rind, if using. Add 6 cups of water (or broth). Add potatoes, reduce heat, and simmer until potatoes are tender about 20 to 25 minutes. Add the beans and the kale and cook until kale is wilted, about 5 minutes. Discard parmesan rind and add the fresh herbs. Season to taste, and add the splash of vinegar.