You are absolutely correct. I have got my e visa a few times now and once had a problem with the Vietnam payment not accepting my Visa card as I was traveling. I had to eventually use a different card to pay. Did they charge me extra? No. They only time they charged me extra was the first time using an online firm and then the tour operator told me that I could use the Government website without paying extra to firms like these.
It is always prudent to check the details in the visa as you say and then double check it and apply again if the details are wrong.
I am sure that what they are doing is not legal and not sure if the Vietnam Government knows about these things. It happened to me a while back in Thailand on the fast track and saw a few weeks later the government had discovered a network of these scams and sacked them. Even in the Philippines, The Chinese Visa scam was busted.

for what it's worth, i've only ever heard of scams happening at SGN, despecially SGN departures, not HAN or DNG.
I kid you not, departure passport control at SGN unironically has long lines because some of the officials are negotiating (scamming) with travellers over completely fictional procedural offenses such as: not bringing the print out of your evisa, thus not being able to validate your length of stay in the country or something (never mind the fact that your passport gets stamped upon entry) or some other made-up reasons targeting vietnamese diaspora visiting the country who don't know any better to scare them and make them pay up.
most cases, if you look like the type of person to not put up a fight when being treated unfairly, unscrupulous locals may try to take advantage of you in some manner.

oh and by the way, VISA ON ARRIVAL (not an evisa) is available for most nationalities arriving by air. you pay US$25 in cash at immigration passport control and they stick a physical visa in your passport and it's single entry and valid for 30 days (maybe 90 now they've changed it)? so even if you've messed up the evisa information in some manner, $25 is the most the airport should charge you for a single entry visa. lots of people make mistake because they use a e-visa website masquerading as gov.vn when it's really a travel agency. and some websites even spoof the name of a reputable travel agency to fool people further. for example, HẢI V�N TRAVEL is a well-known reputable VN/EN language travel agency within Vietnam but many dishonest people take their name and image to do their own travel business.

May I add that sometimes they do not accept the pics that you have with you even though they are passport / visa approved picture. They want to take it from their own camera and there is an extra charge on it. Nothing outrageous but a bit of a hassle.

Data point. Entered @ SGN yesterday and took my wife and myself less than a minute with printed e-visas. I did have to buy 1 visa as VN CXLd our inbound and booked us a day later. I moved it up a day instead but my wife's visa wasn't valid.

Thats interesting as you had to get a new one as I had the same issue last year where my flight was cancelled and � had to move a day early but my visa was good. What happened to your wife's visa? Was it on arrival or the airline itself that had problems with it?

I somehow mistakenly but fortuitously had dated my visa for the new entry date while my wife's was dated for the original. Tip here is to start entry date a few days earlier than planned. The checkin agent was going through the visa very carefully at KUL.

Got it now. Yes I always add two days in advance to my actual arrival date and two days after my departure date.