#1. I'd think that this depends on your nationality. You (usually) can get a visa upon arrival in Cambodia. As for Vietnam, it depends on your nationality, and how often you travelled there within a certain timeframe. So, yes, you might be declined staying on in Vietnam without a visa, which, I presume, would mean a flight home. (As for details and post-Covid updates, why not search Google...
#2. The short answer is 'yes', but you might be lucky to be sent back to the place you boarded at, provided that you don't need a visa to enter there.