Whisk together cornstarch and milk in a small bowl.
Bring vermouth to a boil in a large skillet, and cook until vermouth is reduced to 1 tablespoon. Remove from heat, and whisk in cornstarch mixture. Add whipping cream, salt, and pepper; cook over medium-high heat, whisking constantly, 2 minutes or until mixture comes to a boil. Boil 1 minute or until mixture is thickened. Add Swiss cheese; reduce heat, and simmer, whisking constantly, 1 minute or until sauce is smooth.
*Clam juice may be substituted for vermouth.