too much arguing over nothing, let's debate a few things on not the ownership, but the claims.
1) Vietnam was under Chinese rule for 1,000 years before breaking free in the early days. Later dynasties either are tributary states or allies. One more occupation happened during Yongle, and then Tributary state until France.
Now during this time, either Vietnam had been under China, which would make the islands ours, or a tributary state.
So we at least have a claim IF Vietnam claim happened before Yongle invasion.
This is if we go by Ancient records.
2) These islands were claimed by the French for French Vietnam, and then they were Vessel Vietnam of America, then Communist Vietnam. So from French to Communist, if Communists were to claim these islands then Communist Vietnam accepts it succeeded Vessel and French Vietnam, or else this
So in essence, you are saying a new nation should take over all previous land owned by the previous government.
That means to Vietnam, Xinjiang, Tibet, Taiwan as well as Diaoyu(Annexed by Japan as a result of 1895 war) as well as the islands Philippines holds(Taiwan claims those islands, I don't know the details.) are all Chinese.
This is the view for modern times.
Currently just holding Taiwan, Diaoyu, the Philippines and Taiwan held islands would make China the effective ruler of the east and south China sea.
So even in the most conservative terms it still looks a great deal.