A very interesting online post by Atlas Obscura, examines why 40% of all Vietnamese share the same last name, Nguyen.
Ruan itself might come from an ancient Chinese state of the same name, or maybe from the ancient lute-like instrument also called a ruan. Who knows? Either way, it seems likely that some mid-level Chinese bureaucrat, in seeking to figure out who actually lived in his newly conquered Vietnamese territory, simply decided that everyone living there would also be named Ruan—which became Nguyen.
Read more here: http://www.atlasobscura.com/articles/nguyen-name-common-vietnam