I (white British) have been through countless times - first time 1993, then regularly since 1996. I have never, ever had a problem. So my contribution is anecdotal.
1) I know some "viet kieus" who routinely put cash inside their documents and swear its necessary. They do this before they are asked for anything. These are the same people who love to overtip, and will bribe anybody they can get hold of. A hotel receptionist told me how much she hates them - they'll flash $20 and expect room upgrades, girls in the room blah blah blah. A Vietnam Airlines FA friend of mine is sick of VKs trying the same game - to get upgrades, mobile phone numbers of other FAs etc. Call it a tip or bribe - I think a common thread is that the VKs concerned "love" to pass the cash and think they are getting a favour. At the same time they will claim to despise the system.
2) I also know a couple of VKs who sometimes get "trouble" at the airport and will argue endlessly before finally caving in. One friend didn't cave in and missed her flight. She ended up back in her hotel and got a lawyer involved. Paid more in legal fees than the bribe would have been - but it was the principle. She won't go back.
Hard to know the real balance. But I'm sure (1) can drive an expectation which makes it worse for people in (2).
My VN friends seem in denial about the whole thing - saying it probably never happens these days - "maybe it did in the past". I certainly don't accept this either.