Since you can't combine partners on award itins, your two options appear to be KE and CX. At one time, CX wouldn't allow SGN or HAN as destinations (IIRC it was because those flights were codeshares on VN metal) - it looks like CX runs its own flights to SGN now, so you might be able to get award seats.

Consensus from reading this board would be that CX would be preferable over KE for inflight service, especially in premium classes. KE has angled lie flat seats in J; CX has fully flat, although they are in a "herringbone" pattern which some people like and some hate.

KE's got more west coast gateways - SEA, YVR, SFO, LAX, LAS - upping your chances. CX only has YVR, SFO and LAX. AS/QX will fly you to the international gateway city if it's not SEA.

You should call ASAP and see what you can get. Good luck. I once got DCA-SEA-LAX-(LIM)-SCL-EZE on AS connecting to LA in J on ~2 weeks notice.