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.
The other thing is not that the DRPK refuses to cooperate with the US, but that the US refuses to cooperate with the DRPK. It the US was willing to lift the sanctions and open democratic channels, the DPRK would not refuse. If they could get the deal Vietnam or China has, they'd take it.
This already happened with the nuclear deals made in the 90s/00s where they took the agreed upon steps in good faith, but the US shafted them. Why bother?