Do you need a return or onward ticket to fly to Vietnam without a visa?

Some visa-free travelers make a return-ticket reservation instead of purchasing it; a check can still occur, so this does not guarantee boarding.

If check-in staff do ask for a return ticket and you don't have one, you can just buy a refundable return ticket on the spot to pass the check; if they don't ask, no need to buy anything.

In practice, no one has ever asked for a return ticket when boarding a flight to Nha Trang.

Airlines must comply with the Timatic system: without a visa and without a return or onward ticket, they can refuse to check you in. Bus tickets don't count as proof. Personal checks are rare, but if you already have a visa, they won't ask for a return ticket.

The return-ticket check usually happens at check-in in your departure city, not by Vietnamese border control. They may check just one random passenger or the whole flight — it's random, and without proof of onward travel you may simply not be allowed to board.