I really hated Nha Trang - the party city of VN. Hoi An is considerably nicer/smaller and has considerably better food. Phu Quoc is excellent too, but there's little there except a nice tropical beach. If you want awesome beaches, you should not even be considering Vietnam - stay in Thailand.

Nha Trang has more entertainment/night life, highrises, crime and drugs. It is not too far from a really nice little place - Dalat in the Central Highlands.

In my opinion, the best online resource for SE Asian info is Travelfish.org