Heat oil in a large saucepan over medium heat. Add the onion, celery and thyme; sauté until softened, 5 minutes. Add carrot, sweet potato, parsnip, potato, green capsicum, stock and salt. Add just enough water to cover the ingredients. Simmer, covered, until vegetables are tender, 30 minutes.
Purée half the soup in a blender until smooth. Return the purée to the pan. Stir in milk and heat gently. Stir in vinegar. Sprinkle with bacon just before serving.
Two or three slices of coarsely chopped hot salami or a little pancetta or prosciutto can be sprinkled on the soup in place of bacon.