He inadvertently did not obtain the re-entry permit because he didn't realize it was required. He went to visit his 90-year-old father he hasn't seen in 2 years b/c of covid. So it was a mistake. And he doesn't want to fly out now - only his visa is expiring. It's not idle pleasure travel, he just made a mistake.
One tip that may be useful: try booking a flight out with frequent flyer miles if you have any. I recently booked a "just in case" flight out within my visa period using miles, United, & was able to cancel it 100%, miles were put back on my account & the $20 fee was refunded, no fees at all. Check your flyer miles cancelation policy. (You can tell I haven't traveled much during the pandemic. Apparently airlines have loosened up on cancelation policies.)
No, I stayed overnight. Mainly because I'd never been to Vietnam. Also - flying these days, I think it would be a hassle to go roundtrip same day. Just my opinion. Even for that short flight I had to leave early in the morning to be at airport 2 hours early, & didn't get to hotel until like 5pm. Entry into Vietnam is no problem - at least for me as a US citizen. No Covid restrictions, no pre-flight or post-flight tests required, proof of vaccine not required. (Although I did bring a copy of my vaccine records just in case. I mean, until a few months ago, even certain venues in BKK, like museums, were asking for your vaccine records before entering. But no longer.)
I recently did a border bounce to Hanoi & back from Bangkok. US citizen, I got the 45 day visa exempt entry no problem. Airport immigration did ask me how long I intended to stay in Thailand, I showed them my plane ticket out within the 45 days, & no problem.