Combine peas, water, onion, parsley, and 1/2 teaspoon salt in a large saucepan; bring to boiling. Reduce heat, cover, and simmer 10 to 12 minutes or until peas are tender. Meanwhile, melt butter or margarine in a skillet. Add lettuce and cook just until soft. Add the lettuce mixture to the soup.
Puree half of mixture at a time in a blender container or food processor. Return all to saucepan. Add milk or half-and-half; and vermouth, if desired. Heat through. Season to taste with salt and pepper. Cover and chill 2 to 24 hours. Dollop with sour cream, if desired. Makes 4 to 6 servings.