As stated, this is the general formal requirements from The Ministry of Foreign Affairs, you can investigate further in the source if you like. Pathetic statement “Deep need to be right”. The case is that it’s always someone who proclaims that their personal experience or view is correct and the only reliable source at the end of the day when it comes regulations for entering Thailand is The Ministry of Foreign Affairs and the Royal Thai Immigration Police. Both formally required an air ticket out.
If you travel on 60 days visa exemption you’ll formally need an onward ticket within 60 days.
If you plan to apply for a retirement visa anyway, it’s more convenient to apply for a 90 days Non O visa based on retirement at the embassy at home before you leave and do the one year extension of stay in Thailand.
Then you can apply with income/funds in your bank at home. There’s no onward ticket requirement and you can easily open a bank account needed for your extension of stay.
As you obviously don’t have knowledge of. You can only apply for a METV at the embassy/consulate of your passport nation OR your current permanent residence at date. This is basic knowledge. If you apply they’ll only give you a three months single entry tourist visa. As you just currently is HMCH I guess you already haven’t applied for permanent residence in Vietnam. If so you can apply for a METV.
You’ll get 60 days visa exemption upon arrival’s if you’re entitled, ore else you can apply for a 60 days tourist visa with single entry only. (As you say “currently” located in HCMC).