In a medium saucepan over medium heat, melt the butter. Add onions and saute until tender, about 3 to 5 minutes. Don't allow the onions to brown at all. Carefully add the vermouth and stir with a wooden spoon. Simmer until the vermouth is reduced by half, about 3 minutes. Strain clams and reserve juice. Add clam juice and reduce a little, and then add cream and reduce by half. Add salt, pepper, and sugar to taste. Add potato soup and bring to a simmer. Add clams and cook just until clams are warm. Taste for seasoning, and add truffle oil, if using. Serve in sourdough bowl. Top with chopped parsley.