5 years ago I worked 1 year in Nha Trang. At that time diving was only permitted off Nha Trang and Danang. I know that there is now a dive center on Phu Quoc (operated by Jeremy Steins Rainbow Divers). Since then other areas that I dont know about may possibly have been opened for diving. Halong Bay is extraordinarily beautiful, but from what I heard from local Vietnamese divers there is no good diving. (I may be wrong, but as far as I know there is no dive operator in this area).
I had the good fortune of being allowed to participate in three Vietnamese expeditions cruising the coast line north of Nha Trang searching for new possible dive destinations, but we found nothing significantly different from or better than the NhaTrang/Danang sites. So, my conclusion would be this: the Vietnamese dive sites are few, mostly quite shallow and as far as quality of coral, visibility and abundance of marine life is concerned far from competitive with Malaysia, Indonesia, the Philippines or Thailand.
As I sincerely love Vietnam and the Vietnamese it breaks my heart to say this: if you are specifically planning a dive holiday, expecting 2-4 good dives every day for a week or two, there are far better choices. Nha Trang is excellent for introductory courses or novices looking for experience, but Im afraid seasoned divers will be quite disappointed.
In all other respects Vietnam is an absolutely enchanting country very much worth a visit
