Combine mushrooms, salt, pepper, and water in a large Dutch oven. Place over medium heat, and bring to a boil. Reduce heat; cover and simmer 2 hours. Remove from heat, and cool slightly.
Strain mixture into a large saucepan; remove and reserve mushrooms for other uses. Place soup over medium heat, and stir in beef broth and sauce concentrate; heat thoroughly. Stir in vermouth, and serve soup warm or chilled as an appetizer.