One thing about Vietnam culture is that they gossip like no other. Everything is in the open. As soon as one 'bad' thing happens, everyone knows about it. When one girl came back from Europe and infected a bunch of people, the govt got right on it and quarantined everyone who could have potentially came in contact. Someone turned in a UK friend of mine because they heard him coughing.
After 80+ days, I haven't heard a peep from anyone to suggest that the news isn't exactly what they say it is. My friends are flying around the country like nothing is happening. People are going out and socializing. Look in the news... you see parties and clubs going off. Look at restaurants and night club facebook pages... everyone is out.
If covid was affecting Vietnam, we'd all know, immediately.
So if there was somebody independent doing that in Vietnam, or wherever, then I would take claims of no cases more seriously.