Why would you need your physical credit card for? It's easily set up on your phone or you can remember the number, expiration date and 3/4 digit code on the back. Or just keep it somewhere. I think they completely didn't believe your story. A trip to Vietnam, if lying, should have been 31+ days out. Also since you spoke to quite a lot of people maybe someone remembered you wanting to come back right away. It's an interesting story but perhaps someone found you annoying or red flags went up somewhere. Even a full 24 hours + in poipet May have been a good choice as well. Every single person said stay a day/ night but going back in the morning was a bad move.