Viá»‡t Nam is a variation of Nam Viá»‡t (Southern Viá»‡t), a name that can be traced back to the Triá»‡u Dynasty (2nd century BC). The word “Viá»‡t” originated as a shortened form of BÃ¡ch Viá»‡t, a word used to refer to a people who lived in what is now southern China in ancient times. The word “Viá»‡t Nam”, with the syllables in the modern order, first appears in the 16th century in a poem by Nguyá»…n Bá»‰nh KhiÃªm.
In English, the two syllables are usually combined into one word, “Vietnam.” However, “Viet Nam” was once common usage and is still used by the United Nations and by the Vietnamese government.
Source: https://en.wikipedia.org/wiki/Names_of_Vietnam, http://thefactfile.org/vietnam-facts/