How critical is it to stay in Vietnam while waiting for DTV visa approval?

You are supposed to remain in the country you apply in until the process completes. Will it cause you problems? Maybe yes, maybe no. Why take the chance?

I've seen people being asked for a copy of ALL passport pages to prove they remain in the country, and also people being asked to attend in person for interview

I left Vietnam after 5 days to go to Japan …no problems although could have left sooner….just make sure you take photos of all passport pages before….even blank ones

There is a tiny chance you get called in for interview

I hated staying in Vietnam personally there’s way too much pollution there. Also, it’s much better if you go to different country instead of returning to Thailand.

I guess There’s a higher chance of some sort of rejection if you come into Thailand before the DTV is approved probably although many people are saying that managed to do a border bounce fine

Stay where you are. There are increasing reports of interviews and proof of location being required as additional documents. If you're caught returning to Thailand, you’ll lose your THB 10,000 and possibly your chance of getting a DTV. That’s a small investment for 5 years!

So far, there haven't been any case of people getting into trouble for doing that. But there is a high chance of getting rejected if you get caught.

It will here as people will get hysterical!

Only if they request a copy of ALL your passport pages, a hotel booking to prove you are still there and ask you to attend interview. Up to you as they say.