But if China goes in there ready to proactively hunt ALL Vietnamese assets down and destroy them with overwhelming force, if China plans to strike viciously with sustained forward momentum without considering offering them any chance to withdraw until they surrender, if China makes it clear that a conflict will not be contained and that all Vietnamese territory will be up for grabs if they dare fight, then there is no chance for Vietnam to win anything.
Last big attempt to subdue Vietnam hasn't subdued it, if i remember correctly. More of a protracted and rather indecisive bloody mess in the jungles.
There is a common military legend called "if not for those damn politicians, we would...". If not for those damn politicians, we would have a disaster. There are very big reasons, why we always try to achieve only limited aims, with as few assets as possible.
No, the problem here is exactly what there is no way to crush Vietnam fast. Geography(stretching away from China latitudally) , climate and conditions, messy infrastructure toppled with a very large army, accustomed to fighting in these conditions.
Main vulnerabilty of Viernam - extended coastline, lack of strategic depth in a longtitudal axis - not only isn't as exploitable by modern Chinese navy due to its insufficient forced entry capability as of now, is additionally extremely covered for by extensive vietnamese ASCM(LACM) capability and recently bought submarines. Neither is overcomable by a simple superiority in numbers, being very distinctive guerilla-type assets. So is SAM network.
Worth adding, what all these assets were sbown to be easily transportable between mainland and islands.
Furthermore, while not exactly thw largest, vietnamese airforce is perhaps the most well known example of dispersed use of its assets, and, as far as i know, this experience wasn't thrown away. Nor it can't expand if really threatened: we are talking about quite a populous country here, with more than enough money for a respectable defensive order of battle.
Finally, while it isn't a political topic, but still.
Not only the traditional "friends of China" will make their presence known in every way possible(happened before, assuming lack of direct interference), even actually friendly nations(Russia) will be put to a very avkward position by your "decisiveness". And those weaker will just run away, because the only message you will send to them is one of fear.
Overall: i wouldn't underestimate Vietnam for just its size.