I am impressed with DOT. I didn't like the answer I got, but they responded within 24 hours to my complaint.

Part of my response states "Based on the information you have provided, it does not appear that your complaint falls under one of the Department's rules. The 24-hour rule (14 CFR Part 259.5(b)(4)) only covers flights to and from the U.S. operated by Vietnam Airlines."

I've written back to the kind agent at DOT, asking to have my complaint withdrawn, but I've also asked that the language be changed in the often-quoted document at https://www.transportation.gov/sites...al20130530.pdf.

The original wording that led me to think I had a complaint, as I had accessed the US website of VN, was this (emphasis mine):
"In addition to complying with the requirements under 14 CFR 259.5 to adopt and adhere to a customer service plan, 14 CFR 259.6 requires that each U.S. and foreign air carrier that has a website marketed to U.S. consumers post its customer service plan, which must include a commitment pertaining to the 24-hour reservation requirement, on its website in an easily accessible format."

This is why I felt I had a case at canceling my ticket.

Thank you all again for your advice. I appreciate it!