I don’t think there are any scheduled flights into Vietnam except for repatriation flights for citizens. There may be some foreign experts, business people and diplomats arriving but everyone must spend 14 days in quarantine at their own expense. Things may change in August but no announcement of that yet. Even if flights resume, I doubt the 14 days of requiring quarantine will be lifted anytime soon. From what I have read, quarantine is not in very comfortable quarters.

I think you can start a policy with Pacific Cross up to the age of 75. You may continue till 99, but they will not accept new policies for anyone over 75. Whether it is affordable is a question. Premiums go up with age.

You could just get a tourist visa from a Thai embassy or consulate in the US. That is good for 60 days and can be extended for another 30 days for about 2000 baht at any Thai immigration office. Then if you did a border crossing, you would get 30 days more. That should cover you from late November to March. Integrity Legal has a YouTube video with useful information and also their phone number in the US as well as in Bangkok is listed.

If $100,000 in health coverage is required for tourists to visit Thailand, that would be several times more coverage than is currently required to get a retirement visa, 400,000 baht minimum.