24-hour visa free Guangzhou

Having in mind your presentation, I can just conclude that your itinerary is definitely eligible for 24 hour visa free transit and you do not need any kind of visa. There is no doubt about it.

As far as your last questions is concerned, you are advised to tell to the Vietnamese official to take a look at Timatic. Timatic is the source that matters and everything else is irrelevant. Timatic is used by airport authorities in determining whether a passenger can travel to a certain place or not. There, exactly in Timatic, they will see that your itinerary is eligible for 24 hour visa free transit in mainland China. Also, it will be check-in counter in Vietnam where you eligibility will be checked. It is just important that in the case of ignorance you remain firm and if needed they will even call their Chinese counterparts to check whether you are eligible or not. They will, in that case, receive an affirmative answer.