Pacific Airlines and Bangkok Airways both fly BKK-HCM (I think Pacific is now owned by Jetstar).
Both of these airlines often have promo fares.
At the moment Bangkok Airways have special fares in April - called Delight Fare.
If you go to the websites of these airlines you can book online (pacificairlinesdotcom and bangkokairdotcom)

My husband recently booked BKK-HCM on a special fare with Pacific but ended up on a Bangkok Airways plane as these airlines seem to codeshare.
We've just returned from Vietnam (flew in to HCM City) and had a good time.