Dont get your hope up, at the end of the day China is still at a overwhelming position to keep everything in check in the region, in case of possible conflict between vietnam and taiwan, even if US stand by and see one of their lacky fight another lacky wannabe just like that it is certain that Mainland China wont allow anybody else defeat Taiwan in territorial disputes.
Well though i agree China wont stand by and see its 'province' Taiwan fight Vietnam in case of conflicts over these islands, I doubt Taiwan will ask for help anyway, and even if they did i doubt China will intervene. They might just sit and watch until both have fought each other to exhaustion and if Vietnams comes out on top, then ,Maybe China will intervene just to take over the islands for itself, not 'helping' Taiwan per se(thanks for the correction my firend Götterdämmerung.lol).
Moreover we should note that Chinas so called 9 dashed claim was not created by the CCP, it was indeed created by the current Taiwanese government(KMT)nationalists, and also Taiwan(KMT) still claims even more territory for China than the CCP itself.lol The communist just took over from the KMT, so i Taiwan is to blame for all the crisis in the region in the first place.lol This is something many people tend to forget. That's why i said earlier on another thread that had the KMT won the Chinese civil war and be ruling China instead of the CCP, then i have no doubts that they would have long intervene militarily in the SCS and seized control of all these islands before other countries in Asia had even gained their indepednence/started to make claims in the SCS. A democratic China(under the KMT) close/allied with the U.S will have no problem doing all these/waging war for these islands and even taking the senkakus islands(without much fanfare from our media). Had they won the Chinese civil war then believe me Asia will have looked much more different than it is today. For one Vietnam, Cambodia, Laos and North Korea wont be communist today to begin with, and im not sure Mongolia will even still be independent today.lol Anyway thats history and we cant change history.