The rest of the story

Two happier developments on the "Visa Approval Letter" fiasco front.  First, when we got to Seoul, we discovered they had upgraded us to business class for the Seoul to Hanoi leg of the flight.  The lie-flat seats were great after 18 hours in the air.  Second, apparently, the "expedited processing fee" we paid to get the visa letter corrected covered the visa fee we were expecting to pay on arrival in Hanoi.  I still don't recommend making that particular screw-up a habit.