All three probably to be honest! Nha Trang is going to be a definite visit for beach times, and maybe that island in the Gulf of Thailand, Phoc quon or something of similar ilk
Phu Quoc is really nice but a little quiet, it depends what you're interested in really. Some of the beaches are fantastic, and I'd recommend hiring a moped and biking round as much of the island as you can. I wouldn't expect much to be happening in the evening, but you can get some great food.
On the mainland, Nha Trang has a nice beach and apparently Mui Ne is even nicer, although I haven't been there personally. Hue and Hoi An (in the middle of the country) are probably worth a visit if you have time. I enjoyed the DMZ tour in Hue although I don't imagine it would be to everyone's liking.
Hanoi is a cool city, and is also a good base to do some of the more popular excursions. Definitely go to Ha Long Bay, on a two night trip if possible, and look at Sapa too - I wish I had had the time to go there. There's a great hostel in Hanoi called Hanoi Backpackers (if you're ok with hostels obviously) which runs its own tours is by a long way the best hostel I've ever stayed in. It was about $6 a night and that included free internet and free breakfast if you were up before 10. I think there are two hostels run by the same company in Hanoi, and it's the newer of the two which is the better one. It also has a decent bar and is a great place to meet other travellers.
Edited by White_Shadow at 17:15:48 03-08-2012