Assuming that you don't qualify for visa exemption (because you are getting a tourist visa also before the first trip), you are correct: apply for a new tourist visa from Vietnam.
interesting, which embassy is this? Requiring exit tickets for non-O definitely is something new, it has not been required so far.
Now the requirement was already pointless for tourist visas but at least it had a little bit of merit, but requiring it for long stay visas can borders insanity.
But if the embassy requires it, then work around it. I would avoid losing hard worked for money on something so pointless, so I would book the refundable flight and then cancel again.
Also, which embassy is this? Their cost seems a little higher than usual.
Not really, but, although it sounds even stranger, the reality is even worse. You don't have 24 hours, you have until midnight on that particular day to leave Thailand.
You could instead go to immigrations and apply for an extension of stay that you do not qualify for. You'll get an extension denied stamp, giving you six more days. You still pay the fee which is 1900 THB, so if you don't gain much by those 6 extra days and need a quick return anyways, you could as well just leave on your last day.