Heat large skillet over medium heat. Add mushrooms, onions, and sausage; cook and stir 8 minutes or until onion is softened. Transfer to your Crockpot slow cooker.
Add tomato sauce, wine, and vermouth to slow cooker. Cover; cook on LOW 6 to 7 hours or on HIGH 3 ½ to 4 hours.
Add clams. Cover; cook on HIGH 10 to 15 minutes or until clams open. Discard any clams that do not open.