Pour lentils in a bowl, cover with water, and allow to sit overnight.
Over medium heat, cook onions in olive oil in a large soup pan. Add garlic and celery. Once celery begins to soften, add carrots, tomatoes, bay leaf and vegetable stock.
Pour in drained lentils.
Bring to a boil, reduce heat, and allow the soup to simmer for 30 minutes.
Remove and discard bay leaf and add vinegar. Season with salt and pepper to taste.