What is the issue with China and Vietnam?
Erm.... I'm thinking you probably mean the Spratly Islands whereby there have been slight skirmishes in the waters around there..... heh a slight skirmish in the Gulf of Tonkin caused a massive war. The Spratlys being 400nm from China while only being 83nm from Vietnam. They've been contested for decades if not centuries. Tbh. With potential oil fields in those there waters who gets them gets the $$.
Stranger things have happened. For instance the UK lays a claim to the Falklands which are over 8000 miles away compared to Argentina which is in spitting distance. 2010 oh look oil has been discovered there...... maybe.
But it's simple, money and power.
We unfortunately live in a world where the right thing is not always done and MIGHT IS RIGHT. To hell with the consequences. Most countries are guilty of this sort of behaviour at one time or another when you are bigger and badder than the other guy. Though remember China got its ass handed to it in 1979.
163 years ago a joint task force imposed its might against Imperial (the shadow of) China and got multiple concessions, various opened ports and then some.
Diego Garcia whereby we exiled the natives to build an airbase on their homelands.
Etc etc.... its just power playing might is right BS. I'm not even going to take sides or say who is right or wrong I'm simply not going to go there at all!