From the Dept of State's website, bolding is mine:
Pre-approval for Visa on Arrival: The Government of Vietnam has authorized some businesses and travel agencies to arrange for pre-approval for a “visa on arrival” at the airport. However, some U.S. citizens have reported being charged unexpectedly high fees and additional charges upon landing in Vietnam. The Government of Vietnam and the U.S. Department of State recommend that travelers obtain a visa directly from an embassy or consulate of Vietnam prior to arrival.
My guess is the current visa wasn't valid due to the "typo" and they were going to process a visa on arrival, which, as I read it, you need to have an invitation letter which most companies will charge a small fee for. But since you are standing there, no doubt they took advantage of your plight and gouged you for the emergency visa. It is Vietnam after all. Seems common if it's on the DOS's website. Best you can do is report it to the embassy so they can have a record of it. You're not living until you get scammed in Vietnam...happens to the best of us.
Hopefully you enjoy your trip there, beautiful country and can't wait to go back.