Well it probably would have been best to post it in the DL forum since you want to use Skymiles. Generally speaking codeshare flights are off limits if the operating carrier isn't a mileage partner which VN is not (yet at least).

KE has their own flights to/from Vietnam which is probably your best bet, availability on KE intra-asia is usually pretty good though the times aren't always ideal on their SE Asian flights.

I think NW also does SGN if that's your destination.