What about all those Nguyen last name? They're as common as Smith in the US

In old Vietnamese society, if someone contributed to the court, the emperor allowed that person to adopt the royal last name, a practice much like being knighted.
There are other historical reasons for the prevalence of the Nguyen name. During the Tran Dynasty in the 11th to 13th centuries, many of the families of the prior dynasty courtesans - the Lys - changed their name to Nguyen to avoid persecution.