Place the sausage in a large pot over medium-low heat and cook until it begins to render its fat, about 2 minutes. Add the onion and cook for 2 minutes. Add the garlic and potatoes and cook for 2 minutes. Add the kale and cook, stirring constantly, for 2 minutes longer. Stir in the chicken broth, vinegar and salt. Bring to a boil, reduce heat, cover and simmer for 1 hour. Season with pepper. Stir in the tomatoes and cook, uncovered, for 15 minutes. Divide among 4 bowls and serve.