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.
I see some users talking about Korea as an industrial nation and while that's true it's also only a small part.
The DPRK is still at war with US-Puppet Korea officially. There are military drills every year and the US is committed to its destruction. Vietnam as a territory is whole. They won back their country and have lived relatively unopposed by the US for many years. The DPRK was split in half and is still under active military threat from the puppet state. Almost everyone in the DPRK has family, grandmothers, cousins, aunts, on the other side.
To borrow an analogy from Malcom X, Vietnam has had their knife removed. They have begun to heal from the atrocities committed and are free to pursue relations accordingly. The DPRK has had their knife thrust deeper and deeper by the US every single year, of course they haven't healed. If the US pulled out of Korea, adopted relatively friendly relations, and gave it 40 years then the DPRK would be far more cooperative.
The material conditions are massively different between the two countries so of course they have different policies.