Cook first 3 ingredients in a Dutch oven over medium heat 12 minutes or until onion and celery are tender. Add vermouth, and simmer 1 minute. Add oysters.
Whisk together flour and clam juice in a small bowl; whisk into soup. Bring to a simmer over medium heat. Simmer 1 minute or until oyster edges begin to curl. Reduce heat to medium-low; add half-and-half and next 4 ingredients. Cook over medium-low heat until thoroughly heated. (Do not boil.) Serve with Rosemary Croutons.
Fix it Faster: Use prechopped fresh celery and onion that's now available in most grocery store produce departments.