Review the nearly daily postings about this subject.
Specifically addressing your flight from Vietnam to Thailand. Air line personnel will ask for your onward ticket out of Thailand. Have it available to show them or you will not be allowed on the flight.
I was asked to show this three weeks ago on flight from Danang to Bangkok.
Thai immigration sometimes asks for onward ticket and sometimes ask to see that you have enough money and a place to stay. This has not happened to me but be prepared.
USA citizens are allowed 30 days visa exempt; soon this will be 45 days.