Wipe out the soup pot, add the vegetable oil and heat. Add the bacon and cook over moderate heat until crisp. Remove the bacon and reserve for another use. Add the chopped onion, shredded carrot and green pepper and cook over moderate heat until just softened, about 7 minutes. Add the cabbage and cook, stirring occasionally, until softened, about 7 minutes longer. Add the reserved broth, the potatoes, tomatoes, apple and brisket and season with salt and pepper. Cover partially and simmer until the potatoes are barely tender, 15 minutes. Reduce the heat to moderately low, add the beets and cook until the potatoes are very tender, about 30 minutes longer. Stir in the vinegar and sugar and season with salt.