We are not likely to know what went on before what is shown in this video that may have provoked the Chinese.
Let's take a hard look at several of the frames of the video itself.
From the video, you cannot see any appreciably smaller Chinese vessel near the Vietnamese vessel. In the two pics that follow, you can clearly see another vessel behind the Vietnamese vessel...which may or may not be Chinese. However, in the second picture, it is clear that this vessel, whomever it may belong to, is actually larger than the Vietnamese vessel. The Vietnamese vessel could have been harassing that vessel, we simply do not know. But the differences in size between those two tells you that the Vietnamese vessel was not a mortal hazard to that other vessel. That is clearly not the case with the size differential in what was chasing the Vietnamese vessel down.
.
What you do see, and what is unquestionable, is that for whatever reason the much larger Chinese vessel chases down the smaller Vietnamese vessel, And it does not at all try and intercede itself between the two vessels. No, it is on the near side of the Vietnamese vessel, overtaking it. Why? It shows us. It very intentionally turns into the Vietnamese vessel. Anyone who has been at sea will know what that will do, when a larger displacement vessel turns into the side of a smaller vessel. Its simple physics.
The Chinese vessel does this once, and then, catches up, and does so again. Except the next time, it drives further abreast of the Vietnamese vessel, so as to get better leverage. This time, they are able to turn into it the Vietrnamese vessel far enough forward to use their own displacement to force the smaller ship across their bow and seriously impact it amidships.
The reesult is a fairly forgone conclusion.
The larger Chinese ship intentionally sank the smaller Vietnamese ship. It is clear that it intended to force the smaller ship across its bows because it repeatedly attempted just that and did not cease until it had accomplished it.
In the end, it appears that many vessels stood by to help the Vietnamese sailors who were put into the sea as a result.
IMHO, unless that Vietnamese vessel had done something similarly life-threatening to a Chinese vessel or structure, there was no excuse for what happened here. Even if the Vietnamese ship was near a Chinese oil rig. Even if they harassed a Chinese vessel...unless of course they tried something similar and put Chinese lives at risk.
As stated, that larger Chinese vessel clearly did not simply intercede and place itself between the Vietnamese vessel and the other vessel behind it. It was on the wrong side of the Vietnamese vessel to do that. No, it clearly meant to hit, and turn the Vietnamese vessel.
It is clear that the larger Chinese vessel, for whatever reasons, chased down the smaller Vietnamese vessel on the high seas...which was clearly attempting to flee...and it turned into that vessel and pushed it across its own bow...and sank it.
As I say, we do not know the provocation...but that is clearly what happened, and, IMHO, it cannot be sugar-coated by anything on that video.