EVERY entry to Thailand, including the first one or your 21st entry, is always at the discretion of the Immigration officer you are dealing with on the day you arrive.
Being able to enter Thailand on Visa Excemption is a privilege and not a right or ever garrenteed..
Stick to the rules as a genuine tourist and you should not have any problems..
Try to bend the rules or think you can and you will eventually have problems.
For Thailand you get a free visa exemption entry for up to 60 days on Australian passport.
Nothing to do before you arrive and just present your Australia passport to Thailand immigration and get stamped for entry.
For Vietnam you will definitely need a visa before you travel..
If you intend to go to Vietnam and then to Thailand and eventually return to Vietnam before flying home you will need a 90 day multiple entry visa for Vietnam..
This allows you to come and go from Vietnam as often as you like during that 90 day period.
There are jobs for English language teachers in Thailand but you really need some kind of teaching skills or qualifications for the best chance of a good job..
Don't expect western standard salaries either as these are not high paying jobs by western standards ..
If you are careful with your money you can survive on a teaching salary but that's about it..
Have a look at the DTV visa
"Destination Thailand Visa"
Depending on your financial situation and your longer term plans perhaps it's an option ?
But keep in mind you must be outside Thailand when you apply for a DTV and it can be done at a Thailand embassy in most counties.