Is "Viet Nam" a mistake, a typo, an archaic spelling, a regional spelling, or an idiosyncrasy of the author? I found the word in this book, and I can't really tell what type of spelling it is.
The country was called Việt Nam or Nam Việt along with some other names that have not influenced the two most common English spellings, of which some are no longer in any use.
Việt refers to a region that covers Viet Nam (particularly the north) and a region in southern China where the Việt or Kinh people come from. Nam means "south", hence Việt 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 Việt 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.
At the same time, some people do not remove the space, as it is closer to the original Vietnamese form, and because Việt 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.
Việt Nam is the Vietnamese spelling of the country name, so Viet Nam is a rendering dropping the diacritics while Vietnam drops the space too. Losing diacritics in foreign words is common in English.
France was the colonial power: French renderings typically drop the diacritic below the e but often keep that above the e as it looks like a French circumflex. They are equally arbitrary about the space.