Can I enter Thailand on a one-way ticket with an onward flight to Vietnam?

When you check in a Miami you may be asked for a return flight. The vietnam flight should suffice especially if you have any additional itinerary / bookings there and they can see you are traveling abroad long enough that buying a ticket back to the US doesn't make sense.

@Roger ******

the airline will check your onward ticket. There's no 60-day Visa on arrival as its only 15-days. If your talking about visa exempt ie stamped in for 60 days that the IO's call on your previous history of entering Thailand. Can you provide this detail please?

If the airline check-in agent asks about onward travel, show your flight booking to Vietnam. But as Tod says, Thai immigration almost never checks.

That being said, the immigration officer did ask me for my flight out last time I entered in June — for the first time in over 25 years of frequent travel to Thailand.

First we need to sort out that you as a US passport holder do not get into Thailand on a visa on arrival.

Instead you can show up without having bought any visa to the country and get stamped in free for 60 days on what's called a visa exempt entry <- which is not a visa just a free stamp that let you in for 60 days

Now as far as you winging your way here on a one-way ticket, it is far more likely that your air carrier will have problems with you flying here without a valid visa to Thailand on a one-way ticket.

If you manage to board your flight you won't have any issues once you land here without proof of onward travel.

Check with your air carrier and see if you can fly in without proof of onward travel on a Visa exempt entry.

Best of luck, safe travels