David is quite corrrect about the fishing conditions here. Anywhere easy to get to by boat or motorcycle will be fished out. There is almost no saltwater fish available in the local markets. The freshwater scene is the same, but you can fish at a farm or stocked "pond". Not worth it for a vacation item. Bring your tennis raquet and you wil be very happy. There are some remote jungle locations that require a 2 hour trek (gotta know a local) to get to. The Mekong hac some nice cats in it, but, what you may consider normal size ones (12 lbs and smaller) are rare as they are netted. The monsters evade the nets and once in a while one is caught. I suggest a couple weeks. a supply of chickens and a tuna rod to get one of those. Nothing here bites on flys. Saw some locals using sweet potatoes for bait and doing quite well..ahem. They would get 2 kilos of fish in an hour, 3-5 grams at a time. However, they have to get to the spot eary and sit all day to wait for the evening when the school of micro-fish would pass by. Not worth it mate. Surf pearch found in the USA are bigger than anything wild caught here.
If anyone has info to the contrary, I would love to hear about it.
Oh.. Phu Quoc Island is known for it's fishing, but, I have not been there yet.
As far as Nha Trang goes, FORGET IT!!!. And forget Phan Thiet, Mui Nhe, Vung Tao, etc. as well. As noted above, jsut look at the local markets. Almost no saltwater fish (9"-12" makarel are the largest and even they are a bit rare). The locals have to go to Korean waters to catch fish. The Fishing fleet fills the ocean net fishing while 90% of the boats are still in the harbor. Plenty of tennis courts here though.