Vietnam is classified as Category 2 from the US FAA regarding safety issues. This means (and I stand to be corrected) they can not start any new services to the US. If they already flew there, they could continue flying but could not add new destinations.
I would think…. with the big purchase of 19 B787's that this situation was discussed while the VIP's were in the US recently to take delivery of the first 787, and it will be interesting to see if anything comes of it. Seems to me there would be quite a market to say LAX and/or SFO. I know in the LA area (Orange County) there is a very large and thriving Vietnamese community. My guess is the demand would be there.
Time will tell…
Kap