I generally go to VN a couple of times a month. Always on miles in F. I have other business in Japan so I usually do NH to NRT and then either a) TG to BKK, spend the night in BKK and then catch the morning TG flight to SGN, or b) catch dinner in Tokyo (or stay a few days) and then get the redeye TG out of HND and on to the morning flight to SGN.
CX through HKG is another good option, or BR through TPE although they don't have F.
I used to do LH through FRA when they were flying the 747 to BKK, but that's been downgraded and not such a great ride anymore.
From the East Coast, SQ and EK are good options. Unfortunately EK doesn't have first on the non-stop SGN<>DBX, so I usually go through BKK if I'm using EK.
Unfortunately, there are not a many options for flying F directly into SGN.
Avoid NH J between NRT and SGN. They use horrible old recliners.