Nha Trang has an airport with daily flights from Da Nang, Hanoi and Saigon. You can also come from Saigon by train or bus. If you are traveling from the North, you can get to Nha Trang from Da nang by train. It is also possible to get to/from Da Nang by bus. I cannot recommend the bus, unless you have plenty of time and a low budget. By the way, if you are planning a backpack travel to Vietnam, there is a bus service that offers an open ticket from Hanoi to Saigon (or vice versa) for a few US dollars. With the Open Bus, you can stop in several cities on the way to your destination, stay there as long as you want, and continue the trip. You will find more info about the Open Bus tickets in any travel agency in the center of Hanoi and/or Saigon.