Bring 0.8 liters (approximately 3.5 cups) of vegetable broth and the lentils to a boil. Simmer for about 20 minutes.
Rinse the leek and cut into thin slices.
Peel and cube the celery and carrot.
Pour all the vegetables into the broth. Simmer for another 15 minutes.
Heat the butter in a pot and sprinkle in the flour. Saute briefly. Pour in the remaining broth and stir into the soup until it binds. Add the sausage slices. Season with salt, pepper and vinegar.
Fry the bacon in a dry pan and drain on paper towels. Sprinkle the soup with the parsley. Serve the soup and bacon immediately.