Combine 4 cups water and next 17 ingredients (onion, carrot, celery, lentils, bell pepper, sugar, basil, marjoram, oregano, thyme, pepper, chicken broth, tomatoes, green beans, tomato paste, garlic, and bay leaf) in a large Dutch oven. Bring to a boil; cover, reduce heat, and simmer 30 minutes, stirring occasionally. Add remaining 2 cups water, orzo, and vinegar; cover and simmer an additional 30 minutes or until lentils are tender, stirring occasionally. Discard bay leaf. Ladle into soup bowls; top with croutons.