The only way to get an Extension of Stay based on medical reasons is with a doctor statement and hospital support.
So get him to a hospital and shout out before entry he has Covid, it seems amazing but hospitals, nurses and doctors know how to handle and how to put the patient in an safe area.
Only if you have resident status in Vietnam, otherwise you can not apply there for the Multiple Entry Tourist Visa.
AND the stamps in your passport and boarding passes do show you are not coming from home country, so fooling the system with fake VPN and fake tickets is not the best thing to do.
IF you enter using a valid Non Immigrant visa you get a new stamp with a new term of 90 days of stay.
IF you ask about the 90 day reports for people with long term stay, it is not valid IF you use this visa as you are not allowed to stay longer than 90 days.
Did you buy a Single Entry Non Immigrant O visa, that the answer is NO as in Single it means you can only use it once
OR
Did you buy a Multiple Entry Non Immigrant O visa, than you can enter as many times as you like and get 90 days AS LONG AS you enter before the valid until/enter before date mentioned on the visa.
Later this year and you want to stay 7 weeks on a 30 day stamp?
More than time enough to apply for a Single Entry Tourist Visa and get 60 days on entry without hassle with airline companies or buying fake tickets to show.
You just recieved a stamp at Immigration but they told you shut up or we will shoot you. Questions about stamp and travel are asked on Facebook, not at Immigration office.