I've never said Vietnam's never been one country. I said NV was not
part of the SV. And vice versa. Therefore it's nonsensical to say that SV seceded from NV -- SV has never been a part of NV; neither had NV any legitimate grounds to act as a representative of the whole country, and at any rate, it did not control the whole country.
There were at the time two governments in both parts of the country, so for all practical purposes South and North V-m were, politically, two different countries.
The North never allowed free elections, which was planned as part of the process of unification of the country. With the massive -- military, economic, and political -- support of the Soviet block/China the North then attacked the South -- first by sending masses of infiltrators (Viet cong) and later on, in 1975, via direct military aggression. It's as simple as that -- we, the young, got our history lessons all right (though not exclusively from ZMag, you know.)
In my view, the point is moot (whether it was or was not, strictly speaking, a civil war), but it's at least as permissible to say that it wasn't as it is that it was. Formally, it wasn't. We don't call the 1812 Anglo(Canadian)/American war a civil war. We don't call the Korean war in the 50 a civil war. We don't call Indo-Pakistan wars civil wars. Same thing here.