Listen to the entire clip from guancha/Shilao folks talking China's relationship with ASEAN countries.
They basically just focused on China-Vietnam, since that's the one that seems to be mending right now. Basically, they said it was really hard to negotiation back 20-30 years ago, because people involved in the war were still around.
Then when they got to the 2010s, Vietnam needed to build HSR and picked Japanese. But Japan couldn't deliver, so it's just China now.
And they also mentioned that China has great relations with Laos, Cambodia and Thailand. Vietnam is on an island if it doesn't try to get along with China.
And I think it's also worth mentioning that Chinese coast guard and military have built up so much that Vietnam has no illusions that it can challenge China's naval control in SCS.
So, there is a whole confluence of factors that have led to renewed engagement in military between the two countries. And we will see if China can sell any weapons to Vietnam. There would be a huge uproar from Vietnamese nationalist if they buy anything major.