You are supposed to stay in Vietnam until a decision is made. If you’re not there and therefore can’t prove that you are it’s possible your application will be denied.
You may find that, as with the Bangkok Bank London branch, that it is an investment branch of the main bank and it doesn’t offer normal public banking services.
The London branch of Bangkok Bank does offer a currency exchange service to any Thai bank account but that’s pretty much it for banking services other than investment.
It doesn’t even offer any services to existing Bangkok Bank customers.
it’s not overstay. Overstay is when you stay in the country longer than the stamp you received when you arrived allows.
90 day reporting is done whilst in the country having stayed more than 90 consecutive days. You could be on a yearly extension, not have to leave the country at all, be late for your 90 day report (outside the window of opportunity you described) and get the ฿2000 fine.