@Steve *****
could be, but then you forgot to mention that the visa Vietnam calls "visa on arrival" has to be applied for upfront, unlike the name suggested. Hence Stuart's point that without exemption, a visa is needed to board the flight.
Vietnam has by now revised their visa program and they have also gotten rid of this inaccurate visa naming. You still apply upfront, but it's now called an e-visa, and you can get it directly from a government website unlike the former visa on arrival which required using a middleman. The options regarding cost, duration and entries have remained the same.