Agreed, the thought that this is some huge concocted scam is just plain nonsensical. On my last flight into SGN at boarding there were 2 people with evisas that had incorrect information on them that was put in by them and they were being boarded with the idea that they would need to rectify it in SGN.

Are officers in SGN taking advantage of people mistakes in their visas? Absolutely?
is it wrong? Absolutely.
Is it hurting their tourism? Without a doubt.
But it all comes back to people being completely thorough with their visas before coming to avoid this happening.
The amounts of money you say they are asking for is insane, sometimes it takes some very heated bargaining to get them to a fraction of what they're asking for, but unfortunately they know people will pay.
This is the one thing I despise the most about my wife's home country.