To be clear, I'm not making accusations about Vietnam, I was just thinking about it and found it a little strange.
I can think of some fairly plausible reasons, but just because those reasons are plausible doesn't mean they're actually correct, so I was wondering if anybody had a more concrete historical and materialist understanding of the situation.
Vietnam was under brutal sanctions as well until they took an IMF loan and opened up their economy which let a lot of manufacturing into the country. This helped normalize relations between them and the US. They also united all of Vietnam during the war, so they don't have any major ambitions for expanding territory in a way that would disrupt US capital flows.
Conversely, DPRK is adamant about not opening up their economy or taking any IMF loans and still claims the territory of RoK, which is a huge cornerstone of US capital because of their chip manufacturing. So overall much less friendly to US capital.
Also what supafuzz mentioned about tensions between Vietnam and China because of the tenuous situation with borders and South China Sea means a potential regional ally against a US enemy state.