Bring vermouth and garlic to a boil in a large skillet over medium-high heat; reduce heat to medium-low, and simmer 7 to 10 minutes or until vermouth is reduced to 1 Tbsp. Whisk together half-and-half and cornstarch. Whisk salt, pepper, and half-and-half mixture into vermouth mixture; bring to a boil over medium-high heat, whisking constantly. Boil, whisking constantly, 1 minute or until mixture is thickened. Add cheese; reduce heat to low, and simmer, whisking constantly, 1 minute or until cheese is melted and sauce is smooth. Remove from heat, and use immediately.