In a large soup pot, heat oil over medium heat. Add onions, carrots, and celery; cook and stir until onion is tender. Stir in garlic, bay leaf, oregano, and basil; cook for 2 minutes. Stir in lentils, and add water and tomatoes. Bring to a boil. Reduce heat, and simmer for at least 1 hour. When ready to serve, stir in spinach and cook until it wilts. Stir in vinegar and season to taste with salt and pepper- add more vinegar if desired. Serves six.