Nah Trang was no doubt beautiful before the unobstructed development kicked in, but, now that it has, I’d suggest avoiding. Whilst there are quite a few things to get up to, like the mud baths, the various boat trips and general partying / getting on the lash, its just that is super touristy and below par from the rest of wonderful experiences you can have in Vietnam.
For ideas on how to get the best out of Vietnam, that doesn’t include Nah Trang, see this entry for a 3 week itinerary for the highlights of Vietnam including what to prioritise and tips for the trip.