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.
It's all a part of the USA's strategy to encircle China. Any country that wants to develop wants to normalize relations with the global hegemon. The DPRK has always seaked normalized relations with the USA, Trump visiting the DPRK is a recent example. However the USA keeps antagonizing the DPRK to keep South Korea and Japan in line and keep North East Asia unstable. Vietnam is part of their encirclement strategy in China's south.