I emailed the Hanoi embassy last night to enquire, and this morning received a reply from them with an attached (rather stringent) list of their Non-Imm O Retirement visa requirements:
• 1 Application form for visa (fully filled) and 2 photos (3.5x4.5cm)
• A personal data brief/ a CV
• A copy of bank statement showing a deposit of the amount equal to and not less than 800,000 Baht or an income certificate (an original copy) with a monthly income of not less than 65,000 Baht, or a deposit account plus a monthly income totaling not less than 800,000 Baht.
• In the case of submitting a bank statement, a letter of guarantee from the bank (an original copy) is required.
• A letter of verification issued from the country of his or her nationality or residence stating that the applicant has no criminal record (verification shall be valid for not more than three months and should be notarised by notary organs or the applicant’s diplomatic or consular mission).
• A medical certificate issued from the country where the application is submitted, showing no prohibitive diseases as indicated in the Ministerial Regulation No.14 (B.E. 2535) (certificate shall be valid for not more than three months and should be notarised by notary organs or the applicant’s diplomatic or consular mission).
You can view the original document in full here:
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I'm pleased to know that applying there is an option, but I'll have to decide whether it's worth jumping through so many hoops.