I assume will be applying for a non-o visa based upon retirement that will allow a 90 day stay in the country. There is no 90 day extension you could apply for.
If you will be returning before the 90 days end you can get a re-entry permit to keep the remainder of it valid when you enter the country.
The 800k baht in the bank will still be in the bank and continue to be there to meet the 2 months before you apply for the extension based upon retirement
Having the E after the O caused some confusion (I changed it a A before thinking it was a typo)) so I have removed it since it being a e visa is not that relevant.
You can do the online report that will be due 90 days after you enter the country.
Be sure you write your new arrival date when you complete the report.
Vientiane will issue one for retirement. Cambodia may issue one but not sure about now.
I don't recall any reports of one being done at the consulate in HCMC or the embassy in Hanoi.
There have been reports of Savannakhet issuing multiple entry non-o visas.
Requirements are here. https://savannakhet.thaiembassy.org/th/publicservice/non-immigrant-visa-o-visiting-family-member-of-a-thai-nationality-a-sp?page=5d84a44c15e39c26b400453e&menu=5d84a44c15e39c26b400453f
The embassy in Kuala Lumpur does not issue multiple entry non-o visas. They also want to see 400k baht in a bank.
See: https://image.mfa.go.th/mfa/0/n3gTFT2TOE/NON-IMMIGRANT_VISA_(O)_FOR_DEPENDANT_-_For_dependant_of_the_main_visa_applicant_or_foreign_spouse_or_children_of_a_Thai-new.pdf
Penang in the past wanted to see 400k baht in a bank to issue a multiple entry non-o visa. The funds could be in any bank. Not sure what they are wanting now since there have been no posts about them and their website has about zero info on it.
When applying for a non-o visa at immigration the 400k baht only has to be in the bank on the day you apply and then for 2 months on the day you apply for a one year extension of stay based upon marriage.
It square at Laksi. See: https://bangkok.immigration.go.th/en/courses/muangthongthani-will_be-closed/
This what that page for visa exemptions shows.
"Short-term Visa Extension for Visa Exemption (ผผ.14, ผ.30)."
Vietnam qualifies for a standard visa exempt entry but is also listed on the visa exempt list.
You should be able to apply for a 30 day extension.
See this file and read the remarks about the ** shown on for Vietnam at the bottom page
List of countries for visa exempt and visas on arrival dated October 2021.
If you have a valid visa in your passport immigration will not stamp in visa exempt.
The METV's 6 month validity starts from the day it is issued.
If you were allowed a visa exempt entry that would reduce your total stay you can get from the METV (about 270 days)
They do not show it on their website but it can be asked for when you apply.
See: https://savannakhet.thaiembassy.org/th/publicservice/non-immigrant-visa-o-visiting-family-member-of-a-thai-nationality-a-sp?page=5d84a44c15e39c26b400453e&menu=5d84a44c15e39c26b400453f
"Children aged below 18: - Vaccination not required if travelling with parents / legal guardianAll age group may travel alone (unaccompanied) if fully vaccinated"
It is shown under this heading on this webpage. https://consular.mfa.go.th/th/content/thailand-pass-faqs-2