Just for future reference fior anyone stumbling across this thread, be careful to book a flight that is operated by Vietnam Airlines and not a codeshare operated by Pacific Airlines, as you do not get your luggage through checked if the first sector is operated by them.

We did it, with eyes wide open thankfully (I.e. assumed the worst), last year DAD-SGN-SIN and had to collect our luggage at SGN before moving across to international terminal for the VN flight.