Level of testing / surveillance is not clear.
See
https://www.worldometers.info/coronavirus/covid-19-testing/ They have run about 2400 test which is 24 per million in population (on a par with US which is wretched). Italy is 1,000 per million, Hong Kong is 2,000 per million, South Korea is 4,000 per million. Vietnam may be going a good job but they are not testing enough by a factor of 10 to 100 to be sure.