My favorite carrier is Asiana of Korea for Vietnam travel. I've used Asiana for 6 trips and EVA for 1 trip - Asiana wins for more comfort and service. Remember that VN has more international airports than just SGN. You might consider an open jaw into SGN and out of DAD or HAN. I'll be there at the very same time, except that I'm spending most of my time in my favorite city - Hanoi. I'm flying Asiana economy and I've paid $736. roundtrip from Seattle (Asiana gives me a hotel room on my return because my transit time in Seoul exceeds 8 hours). I've never traveled on a more pleasant airline!

(only major problem with Asiana is their website - I book on Orbitz or Priceline)