The most popular last name in Vietnam

If over 80% of American has their last name is Smith, in Vietnam, the most popular last name is Nguyen, accounting for almost 40 % of Vietnamese population. This difference illustrates something very weird about last names: they’re a surprisingly recent creation in most of the world, and there remain many places where they just aren’t very important. 

The existence of family name in Vietnam 
The existence of last names in Vietnam dates to 111 BC, the beginning of a lengthy thousand-year occupation of the country by the Han Dynasty in China. (There were a few short-lived attempts at independence before the Vietnamese kicked the Chinese out in 939 AD.) Before this time, nobody really knows how the Vietnamese handled names, due to lack of written records. In fact even the name “Vietnam” originated from the Chinese; “Viet” is the Vietnamese version of the word the Chinese used to describe the people southeast of Yunnan Province.
It is likely that the Vietnamese, prior to Chinese domination, did not use last names, (or family names, which we should call them, given that in Vietnam and many other places, this name does not come last). This does not make them unusual at all. Prior to the 18th century, much of the world did not use family names. More common would be what’s called a “patronymic” name, meaning your full name would literally translate as something like “Steve son of Bob.” Patronymic names refer only to the generation immediately before and remain common in much of the world, especially in Scandinavia and the Middle East. (Keep an eye out for “surnames” ending in “-sson” or including “Ben” or “Ibn.” Those are patronymic names.)
The entire idea of a family name was unknown to most of the world unless you were conquered by a place that used them. Those conquerors included the Romans, the Normans, the Chinese, and later the Spanish, the Portuguese, the Germans, and the Americans. It was the Chinese who gave Vietnam family names.

How popular “Nguyen” is in Vietnam?
Indeed, Nguyen is the most common surname in Vietnam -an estimated 40 percent of people in the country (and the Vietnamese diaspora) carry the name, according to Vietnam's Tuoi Tre News. Given that the global Vietnamese population totals about 94 million people, this means that some 38 million of them answer to "Nguyen". 

The raise of “Nguyen”
The surname “Nguyen” is believed to have originated in the Chinese surname "Ruan" (in the Mandarin language) or Yuen (Cantonese), owing to China’s long domination over Vietnam. Periodically, during both periods of Chinese rule as well as Vietnamese dynastic power in Vietnam, the name Nguyen was either forced upon the public (sometimes by threat of violence and even pain of death) by new regimes or adopted by the subjects voluntarily for various reasons.
By the early 19th century, the Nguyen Dynasty seized power in Vietnam (which unified the country), prompting yet another wave of surname changes and adoptions. That dynasty, which "awarded" their surname to many people, ruled the country until the end of World War II. Esther Tran Le, a Vietnamese-American journalist based in New York, said that the name ‘Nguyen’ was the last name of the last dynasty of Vietnamese emperors. “Apparently the last King's name was Bao-Dai, but his real name was ‘Nguyen-Vinh-Thu,’ she said. “Many of the Vietnamese [peoples’] last names derive from the former Emperors' last names.”
As such, the surname Nguyen does not necessarily denote one's regional origins, class or even lineage. “The surname itself has nothing to do with social class, unless it's combined with, say, Phúc which is a combination of [the names of] the last royal family,” said Dr. Nguyen-vo Thu-huong, associate professor in the departments of Asian Languages and Cultures and Asian American Studies at University of California, Los Angeles.

Presumably, as Vietnamese communities in Australia, France, Canada and the United States increase in number and political influence, we may see a new wave of lawmakers and other prominent public figures with the ‘Nguyen’ surname.