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.
If the US dropped the sanctions regime against the DPRK, I am sure that they would also compromise with western capital to facilitate investment/modernization like China and Vietnam did. Just like with Cuba, it is US intransigence and maintenance of war footing (the illegal blockade for Cuba, and the US military occupation of south Korea against the DPRK) that prevents cooperation.
Multinational mining companies have been salivating over North Korea's natural resources for decades and would jump at the chance to get access the second sanctions were lifted.