Anh Do explains in The Orange County Register why so many Vietnamese are named Nguyen (commonly pronounced win in the U.S.).
Most Vietnamese have the surname of one of 16 royal families who ruled their homeland. In chronological order, they are Thuc, Trung, Trieu, Mai, Khuc, Ly, Phung, Kieu, Ngo, Dinh, Le, Tran, Ho, Mac, Trinh and Nguyen, as in Nguyen Bao Dai, the dynasty's last emperor, who abdicated in 1945 and who reigned before communist forces took control of North Vietnam in 1945.
During his rule, officials gave loyal subjects his name, while many criminals made the switch to avoid prosecution, according to Wikipedia. Through the centuries, a family might have adopted a new identity when new royals ascended to the throne, their rise achieved by force or political manipulation. And since Nguyen was the most recent, it's more plentiful. So plentiful, in fact, that some estimates place nearly 40 percent of Vietnamese with owning the moniker. [Link]