Posting a lesson learned so that someone else doesn't get shocked by this. You must have the credit card used to purchase the tickets with you when you check in.
I purchased my Vietnam Airlines tickets online using a credit card. About a week later it was necessary to close the account I used due to theft (nothing to do with Vietnam Airlines). Capital One issued me a new credit card with no problem. When I went to fly out of Hanoi to Bangkok I produced my passport, the receipt from online, and the e-ticket that had been emailed to me. The clerk asked for the credit card used to make the purchase. Obviously, since it had been cancelled I didn't have it, only the new one. I was refused boarding, and had to purchase a new ticket in order to travel to Bangkok.
After purchasing a new ticket, I was given an email address and told to email them and request for a refund. I emailed and was refunded the money in 14 days.