There are a lot of US passengers in & out of SGN on daily basis: BR - 2x, CI - 2x, JL, NH, KE, OZ, CX - 2x, PR, UA, CZ, SQ, TG and probably a couple of others that I missed. It's pretty hard to imagine that VN could fail to fill a daily flight IF they can work out codeshares with US airlines to feed their gateway. I think they already have an agreement with AA.