Seems just a bit high. I’m not saying the Russkies haven’t suffered losses, even large losses, but 400K men is just nuts. The NVA facing American firepower for 8 years didn’t suffer losses like that
The NVA (PAVN) suffered 1,081,000 dead and missing, 1960-1975, according to the Vietnamese government. Its unclear whether the Vietnamese government includes VC casualties in this.
https://en.wikipedia.org/wiki/Vietnam_War_casualties