"The country was called Viet Nam or Nam Viet along with some other names that have not influenced the two most common English spellings, of which some are no longer in any use.
Viet refers to a region that covers Viet Nam (particularly the north) and a region in southern China where the Viet or Kinh people come from. Nam means "south", hence Viet Nam means the south part of that region. Historically this wouldn't cover the very south of modern Viet Nam, but then considering that the etymology of Dutch means "Germanic", it's not like names and borders remain neatly aligned throughout all of history.
In Anglicising, some people merge the two words into a single word, as it is after all almost always used as a single name in English, and we are not familiar with the words Viet or Nam. Comparably, one could imagine a language transliterating "North America" as a single word, if they weren't familiar with "North" or "America", and so on. Some also favour the single-word form because the official translation of "Cong hòa Xã hoi ch? nghia Viet Nam" (the offical name of the current country) is "Socialist Republic of Vietnam" which uses the one-word form.
At the same time, some people do not remove the space, as it is closer to the original Vietnamese form, and because Vi?t does appear without Nam in the names of some Vietnamese organisations, etc. that have also been relatively well known among English-speakers outside of Viet Nam, particularly the Viet Minh and Viet Cong."
and
". . .This legend symbolizes the importance of uniting the two main geographic and cultural areas of Viet Nam -- the mountains (representing the north) and lowlands (representing the south) in forming one united country. It is a theme that gets played out repeatedly in Viet Nam's history and is also symbolized by the spelling of "Viet Nam" as two words, rather than one. In fact, spelling Viet Nam using two words has a long tradition and is in keeping with the country's pre-colonized history. It wasn't until Viet Nam was colonized by France that its name was shortened to one word. . ."
You said in a few words what the articles said in a gazillion!
