Imo, they wouldn't expect the return ticket to be any longer than 90 days from the date of travel unless your going to show a full travel itenary inc the Vietnam flight tickets to cover the period out of Thailand before returning.
The OP says 2 months in Thailand and a 'month or 2' in Vietnam before returning to Thailand for a flight home. I'm not certain they will accept a return ticket 4 or 5 months ahead without some sort of plan when asking for a single entry?
Perhaps they would expect an application for a METV?
unfortunately, nobody can tell what the reaction is likely to be at immigration on any given day. IMO, with your history, you won't have a problem BUT it's not me stamping you in 😀
As a middle aged couple looking to live here, I'm certain you could show the required £5000 for the previous 6 month's each in a UK bank account? I'm also certain, the Thai Embassy, in your circumstances, would let the employment letter slide.
Give them a call on the numbers and times in the enclosed link
Here is the answer guys. Seems once again they have changed what they require from a month ago.
I have no idea what 'Normally' actually means or 'Checking on a case by case basis' It's even more bizarre that if you attend the Embassy after applying with online E Visa, you need to supply the documents but not if you post after applying by E visa 🤔