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 Vietnamese refer to Viet Nam as Vietnam when they write in English doesn't mean that is the correct way. It just means that it is more commonly used. English is a second language to most Vietnamese, when they learn English, every single piece of writing they found refer to Viet Nam as Vietnam, so they think it is the "English translation" of Vie^.t Nam, gradually, it became a habit. Proof: The same thing happen to any big city in Vietnam, Saigon, Hanoi, Danang,... But NOT for Ho Chi Minh City, Ia Drang, Hai Phong,... Somehow, English speakers decided that Vietnamese shouldn't have space when referring to a location.