Vietnam had a combined North and South pre-war population of about 36.5 million according to macrotrends.net. If Vietnam's estimate of 2 million civilian dead and 1.1 million military/militia dead were true then adding ARVN losses on top of this, it would be a bit over 9% of the entire population that suffered a violent death due to the war.
Could that many people have died? From what I’ve read the population was larger at the end of the war than during the war. I know there was a small decline in acceleration, but wouldn’t it have been much more significant if 9% of the entire population died violently.
https://www.macrotrends.net/countries/VNM/vietnam/population
I would trust what the US statistics were because the secretary of defense, McNamara at that time (MBA Harvard graduate) didn’t know how to calculate if they were winning the war or not, so he relied on KDR (kill death ratios). We know they included civilians and it wasn’t completely the VC or NLF but close enough