Both Vietnam and China have a track record of covering up disease outbreaks. They also both use homegrown tests for COVID-19. It is not unreasonable to discount their figures without independent verification.
There's over a billion people in China, with an extensive diaspora that keeps in touch with family back home. The world is connected, and we aren't living in 1929 anymore (Where your entire extended family could have starved to death, and you wouldn't hear about it for a decade.)
What exactly do you think is happening there? They've lifted the lockdowns - and if the virus were still circulating through the community, it should have infected tens of millions of people by now. Do you think that millions of people are getting sick and dying, and their overseas relatives, who are frequently in touch with them, don't know about it?
They've tried to suppress COVID information, when only thousands of people were infected in January, and... Everybody still knew about it.
Do you think they are somehow orders of magnitude more successful at doing so in September, while secretly having millions and tens of millions of cases?
Or is the more plausible explanation that they probably don't have millions and tens of millions of cases?
Or, alternatively, do you think that they only have tens and hundreds of thousands of cases (Few enough to suppress information on), and somehow, despite the lockdowns being lifted, they aren't ballooning into millions?
People thinks the CCP is some omnipotent organization and can suppress any and all information. If that's the case, then how do we even know about the Uyghur detention camps?
So there are no more containment measures in China, not even masks are needed. However, the cases haven't spiked in the 2-3 months since the lockdowns had ended.
If China was covering up the disease and still had thousands of cases, then by now there would be millions of cases. However there aren't, I have friends and China and they don't know anyone who caught the virus in the last months. There haven't been secret lockdowns either because they would be easily noticeable.
As a consequence, we can come to the conclusion that the virus has essentially been eradicated in China, and while I don't know anyone in Vietnam I'd imagine it's the same.