I'm thinking it may have been hijacked with the intention of doing a 911 type attack somewhere in the world where they would have to fly off-course to get there but either blew up (due to passengers not cooperating) or crashed hijacker-pilot error somewhere in another area such as the Indian Ocean or South Pacific. All speculation of course.
I doubt it went down in Vietnam or anywhere else over land. Villagers would have heard or seen something.
This situation reminds me of the Jihad Falcon plot that Adam Gadahn was taunting authorities with a couple of years after 9/11.