Easily...The concept of 'sovereignty' rests upon respect, meaning that it is not enough that you claim to be a 'sovereign' over a territory, but that also others must respect that claim as well.
Sovereignty (Stanford Encyclopedia of Philosophy)
What is this? You pulled this definition from the Internet without a clue of what the reality out there is ... to refute me!? You must be desperated
Let's examine it okay ...
Sovereign authority is exercised within borders - islands are outside of borders ... for grap
but also, by definition, with respect to outsiders, who may not interfere with the sovereign's governance. - China respects Vietnam, is she not? Has China invaded the islands that Vietnam exercised its sovereign authority within borders of the islands and controlled by Vietnam?
So, what is your point or should I say "pointless"?
So if China does not respect Viet Nam's sovereignty over the islands, it does not matter if China has absolute control over some of them, China's claim of sovereignty will not be respected, hence, China does not have sovereignty over them. The respect works both ways: I do not interfere with your borders. You do not interfere with mine. If that respect exists, sovereignty exists.
China does respect Viet Nam's sovereignty over some of the islands that Vietnam exercised its sovereign authority within those islands' borders. What else do you want?
You're right:
You flunked.