Scrub clams and place in soup kettle with vermouth and water. Cover and simmer until shells open. Remove clams with slotted spoon. Strain liquid through double thickness of cheesecloth and set aside. Chop clams and set aside. Clean kettle and fry bacon in it until crisp. Remove with slotted spoon and set aside. Saute onion in fat until golden. Add reserved stock and potatoes. Simmer, covered, 10 minutes. Add clams and simmer until heated through. Stir in cream and reserved bacon. Season with salt and pepper. Heat and serve.