This doesn't seem to work well. The results from SEA -> Vietnam (SGN or HAN) and the results were empty, but if I search for SFO -> SGN. I see flights from SFO->San Diego -> Japan -> SGN, but if I do San Diego -> SGN, it returns no results.
Hmm strange, my server is getting pummeled today with all the traffic from HN so it could or just been temporarily down but I just tried it myself right now and I am getting results for SEA -> Vietnam starting at US$29 from DLI to SGN. Give it another try and let me know if it doesn't work.