Anyone flown to Vietnam for the purpose of returning for a 45 visa exempt entry, and just stayed in the airport waiting for their same day return flight to Thailand ?
Or have you got to exit the airport and get stamp in your passport.?
TLDR : Answer Summary
It is necessary to exit the airport and get a stamp in your passport to qualify for a new visa exempt entry into Thailand after flying to Vietnam. Simply staying in the airport will not suffice. Additional considerations include the need for a visa prior to departure for those whose countries do not have arrival exemptions, and prior travel history may lead to extra scrutiny upon returning to Thailand.