Yes, that's how it is written in Vietnamese. To oversimplify: Vietnamese words are a collection of single syllables that are always separated by a space when writing.

"Viet Nam" is also, actually, the "official" English way to write it. (Check how the UN puts it on all their stuff.) However, most Europeans don't do that in their languages, so it usually gets written as Vietnam even by Vietnamese when they're writing European languages.