This has never been possible since a conversion from a tourist entry stamp to a Non-O based on retirement requires a Thai bank account with 800k (this is not just a requirement for the extension)
Agreed, enter on a Non-O, open a bank account and transfer 800k, get a re-entry permit for your Vietnam trip and, on your return to Thailand and after your 800k has seasoned for 2 months, apply for your 12 month extension then get a re-entry permit when your 12 months is inked in for your return later next year.
if anyone is on a fake or fraudulently obtained visa then fair enough, anyone on a legitimate DTV will be fine for 5 years without proof of funds at the border at re-entry