Focusing on the "Vernacular" (everyday language):
* Dante Alighieri: His *Divine Comedy*, while written in a refined form of Tuscan, is considered a masterpiece of Italian literature and helped establish the Italian language. He used the language of the common people, but elevated it to an artistic level.
Focusing on "Masterpiece" (high artistic quality):
* Leonardo da Vinci: While not strictly literary, his paintings like the *Mona Lisa* and *The Last Supper* are masterpieces of the Renaissance, and were created during a time when Italian was a vibrant and evolving language.
* Michelangelo: His *David* and the ceiling of the Sistine Chapel are considered masterpieces of Italian art and were created during the Italian Renaissance.
Focusing on Both:
* Niccolò Machiavelli: His *The Prince*, written in elegant Italian, is a masterpiece of political philosophy and a significant contribution to the Italian language.
* Giovanni Boccaccio: His *Decameron*, a collection of stories told by a group of people fleeing the Black Death, is a masterpiece of Italian literature that uses vivid language and everyday situations.
Ultimately, the answer depends on your interpretation of "masterpiece" and "Italian vernacular."
To help you narrow down the answer, could you tell me more about what you're looking for? For example:
* What time period are you interested in?
* What kind of masterpiece are you looking for (literature, art, music, etc.)?
* What aspects of the Italian vernacular are important to you (spoken language, written language, regional dialects, etc.)?