Sure, people have been doing visa runs for years in Vietnam, but what's to stop immigration from denying entry at whim? Maybe it's a small risk but a risk nonetheless. I knew an English teacher in HCMC who always carried that in the back of his mind. Perhaps less a worry if one is not working?
During the p(l)andemic VN cracked down hard on revolving door foreigners, even shortening the cycle to 30-day visa runs. Now it seems to be business as usual with 90 days once again and seemingly automatic extensions. But being a Socialist (Communist?) country things often change overnight without warning.
It's impossible to open a bank account in VN without a Work Permit. There may well be agents in VN who can grease the right palms to get these things sorted.
Would be nice if VN introduces a retirement visa. It really is a lovely country, great food, cheap beer, gorgeous women and quite liveable.
There is a VN sub forum here on AN: https://aseannow.com/forum/343-vietnam/