Cook the potatoes a little over half way before chopping. (I microwave them on high for about 5 minutes.).
In a large soup pot, heat oil over medium heat. Add onions, carrots and celery. Saute until onion is tender. Stir in potatoes, garlic, oregano, basil, and pepper. Cook for 1-2 more minutes.
Stir in water, broth, tomatoes, tomato juice, and bay leaves. (The liquid added should total 8 cups.) Bring to a boil. Add lentils and return to a boil. Reduce heat and simmer uncovered for at least 1 hour.
Cook frozen spinach separately, by the package directions, or skip this step if using fresh spinach. Add the spinach and vinegar to the soup in the last 5 minutes of simmering.