I've had this question for a long time but it's very subtle and not easy to correctly express. I don't want the question to sound like "Why are so many Vietnamese people morally corrupt?" because it's not really want I want to say. So here's a recent example:
I started having private Vietnamese lessons with a local woman in Rach Gia who's a primary school teacher and also English teacher on evenings and week-ends. We have a deal for private lessons, I pay her 1M for 8 hours a month. In Rach Gia, that's quite a good "salary". Today, she asked me if her best female friend, a Westerner, could join me in classes. She did not offer to lower the price or extend the hours, so this meant that I'd end up with this total beginner (knows zero word of Vietnamese) in a one hour class which was supposed to be private.
When I told her "no", she said "Okay, I knew you've paid for these private lessons so I wanted to ask you first".
Well, from her answer, I knew the reason was not because she was clueless about the lack of "etiquette" such behaviour has. She knew it was not something that was "okay" but she asked anyway. I am 100% confident that if it was a Vietnamese citizen paying for their kid's lesson, she would never even have entertaining asking to have that class shared with someone without at least compensating somehow.
I know most Vietnamese people know right from wrong. Why is it that they'll sometimes try to pull off something like this? Is it because as a Westerner I'm supposed to be "so nice" that I wont' dare say no? Because she's a beautiful woman and she thinks I'll only think with my other head? Maybe because I simply look so stupid? Heck, truth hurts but I'm ready to accept it