Saute onions, garlic, carrots, and celery in heated oil in a large kettle for 5 minutes. Add water, lentils, bay leaf, salt, and pepper. Bring to a boil. Lower heat and cook slowly, covered, about 30 minutes, until lentils are just tender. Add frankfurters and cook another 10 minutes. Remove from heat and stir in vinegar. Remove and discard bay leaf. Serves 8.