What should a Vietnamese citizen do to avoid issues with re-entering Thailand after multiple visa-exempt entries?

Your entry to Thailand with VISA on Arrival is only accepted 3x in 6 months. If you're a businessman, they would require you to have the multiple entry Visa for the countries safety and your convenience.

If you're travelling to and from Thailand that often, you're not just a regular tourist. And that VISA on arrival is a TOURIST VISA. Therefore they asked you to have a certain VISA that fits your purpose here in Thailand.

Pre-Covid, I observed how Vietnamese and Lao nationals were all being driven through the Phu Nam Ron border crossing between Kanchanaburi and Myanmar, to renew their stamps on an in/out basis. All of them were working illegally in Thailand. Funnily enough one fluent Thai speaking Vietnamese lady pulled out her work documents, which gave me a chuckle, considering she could have been deported and blacklisted for working in Thailand without a work permit.

The reason why she wasn't is simple: corruption. The visa run vans pay a bribe to immigration to allow this to happen, but it could that this service is no longer running in the way it used to.

Not a single Thai national entered the country at that time, it was 95% border runners.

Avoid coming in via Cambodia, as officials there are notorious for giving Vietnamese nationals a hard time. Arriving from Laos is better.

This is normal. You may have "big trouble" next time, or you may not. Depends on the official. Also depends on the port of entry: Suvarnabhumi is notorious for having strict immigration officials. Don Muang MIGHT be better (not sure), provincial airports like Chiang Mai less strict, land entries even less strict.

By the way, getting a tourist visa is no guarantee of entry. If you want to stay in Thailand long term, you'll be best getting a long term visa.

last year I entered 8 times visa exempt. never had a single question. Most importantly each time i left the country for more than a week.

Go by land border next time. ASEAN citizens usually don't have problems.

Even the limited 2 times per year does not apply to them.

Listen to the immigration lady. She’s telling you your history of entering Thailand with no visa is wearing thin. Although Vietnam has a special arrangement with Thailand for entries that doesn’t mean as a Vietnam citizen you can enter any time or as many times you want.

At some point (and by the sounds of it on your next attempt) they may deny you entry.

Get a visa that fits the purpose of your intended visit to Thailand.

🎣🎣🎣

How long did you stay outside Thailand between your different entries?

This happened to us a couple of days ago! We were pulled to one side and they said we had to book a flight back home otherwise we couldn't come back into Thailand, we are now in the process of getting our visas, if you are planning on staying in thailand for longer I would advise you to get a visa too

No country in the world will let you come in and out as many times as you want. Your permission to enter Thailand is at the discretion of the immigration officers. They are telling you that you have spent too much time in Thailand on visa exempt entry, and you either need to get a visa or stay home for a long time before returning.