We took a sleeper bus from Hanoi to Hoi An and then to Na Trang. It was a good way to save time if you don't mind a blaring horn all night and missing on coming traffic by inches. I was just waiting to slam head on into another vehicle. It could be because I was right near the front of the bus, but it sounded like the horn was literally right inside the bus. We usually travel by train but decided to take the bus because it was faster and we heard the train in Vietnam is quite slow.
If budget is a major factor I guess sleeper bus is the best option, just don't stay near the front or the back (if you get car sick easy). The flight from Na Trang to Ho Chi Minh was fast and convenient, but of course more expensive.
Definitely check out Hoi An... it was the highlight of our trip. about 8 years ago
Vietnam Airlines has very cheap flights between all the major cities. We will arrive in Hanoi next Thursday (5/27). A flight from Hanoi to Da Nang is about US$55. I believe there is no airport in Hoi An, so you have to take a bus, taxi, or private car from Da Nang to Hoi An, about an hour ride. about 8 years ago