they have upgraded, redone and fine tuned that system, most likely during process they detected your account being idle and blocked it. Best send them a message, or you try to log on again in few minutes.
I have never seen a Vietnamese with a METV, but it is possible. If teh person plan to do visa runs for 30days, please be aware that Vietnamese are not unlimited anymore, just TWICE in a calendar year and for visa application the Vietnamese have to return to home country.
just had a look and the online appointment system is open for booking up to Friday 31st May. I assume that tomorrow or within a few days they will release the first week for June to the public.
a friend of my son was stopped too, had a new TR from Phnom Penh, money in the bank, but no cash, they send him back to Phnom Penh. I do not understand the world anymore. Now he will try at another border.
most likely the OP is on extension of stay based on retirement. A retirement visa does not exist. Do the TM30 thing first, than 90day reporting. And before the extension expires do a timely renewal.
legally there is no limit, however after a couple of times you will be grilled and warned to get a proper visa. BTW it is not Tourist visa it is the visa waiver program you are asking about.