Saute onion and celery in oil in a 5 qt. pot until soft. Add potatoes, carrots and stock to cover. Bring to a boil; reduce heat and simmer until vegetables are nearly soft, about 15 minutes. Add beets; cook 15 minutes (or about 3 minutes, if beets are already cooked). Add vinegar and seasonings; cook 1 minute. Garnish with yogurt or sour cream for more flavor and nutrition.