The ancient land records stored at Hue prove that for centuries Viet Nam has claimed sovereignty to islands off its shore.
Regardless, territory belongs to whomever will either fight to take it or fight to defend it, just as was always the case going back the earliest day of getting access to the watering hole and hunting grounds.
There is no point in history where national borders are magically fixed and immutable.
I thought more along the lines of their both being communist countries. Not even that type of solidarity prevents them from going after each other.