One other piece of information most are not aware of when attempting a flight out of Thailand to Laos. Laos has a very old rule of denying boarding on a flight to Laos if you have any previous Thai overstay stamps in your passport. Certainly, Laos airways still uses it for denying boarding on flights. No idea if other carriers still apply it. It doesn't apply at land borders.
I doubt you would be returned to Vietnam due to the way the time limit on visa exempt and Tourist visa system operates there. That could well be a case of being returned to your country of passport.
Easy enough to get a single entry Non O in the UK for retirement provided you can supply the documents they ask for.
Not forgetting you'll need to apply by the London Thai Embassy E visa website.
You can use a consulate at Liverpool, Hull, Edinburgh, Cardiff or Dublin but you need to email your documents for pre approval from London then make an appointment. There is no postal services via the consulates in the UK or Ireland.
Non-Immigrant Type O (Retirement (pensioner aged 50 or above with a state pension who wants to stay in Thailand for no longer than 90 days) - single entry only)
Financial evidence e.g. A copy of pension statement if the applicant is a pensioner, or a copy of 1-month bank statement showing your income from pension, or 3-month bank statement of at least £10,000
Confirmation of legal residence in the UK or Ireland
So as to not waste your time or money, I've yet to read of an airline operating from the UK and particularly out of Heathrow, that are interested in Bus tickets. If they ask, they want flights that can be verified on a computer terminal.