I've been to Vietnam quite a bit but never dived there (since we now have kids in tow) but these look pretty good resources:
Whale Island: http://www.divevietnam.com/whale2.shtml
Island of Phu Quoc (which is lovely and still surprisingly unspoiled) http://didulich.net/english/?page=news&menu=detail&groupid=15&newsid=968
Quote: "Nudibranch Gardens: At the deeper parts (around 11 meters), you’ll find a range of large boulders, and in the shallows, a beautiful coral garden. Besides a large population of nudibranches, you'll also find bamboo sharks, giant puffers, crustaceans and blue spotted rays." almost 8 years ago
The three best places are: Cham Island near Hoi An Da Nang, Nha Trang Bay and Phu Quoc Island.
Here is a trip for reference and you also can get more details from them.
http://www.customvietnamtravel.com/vietnam-tours/luxury-tours/vietnam-sun-sea.html about 5 years agoAnswered by Phuong Nguyen via Travellr.com