They see China and Vietnam setting aside their differences and working together and they want to make damn sure none of that happens while they're around.
Did you hear about the recent visit of Xi Jinping to Hanoi? There were many people waving Chinese and Vietnamese flags. I heard from a taxi driver that the roads were very blocked on that day too. He also said, when asked about whether he prefers china or the US, that he prefers china. Do you really think they would side with a country that relished in bombing them north to south, as well as spraying oncogenic agents? Versus another socialist nation that has committed to working together in development?