Public bus
There is an extensive network. But they are slow, stowed with people and luggage, so not comfortable at all. In fact, the government discourages travellers taking the public buses.
"Open tour"
Most low budget tourists prefer the so-called "open tourist" bus ticket to travel through the country. The prices of these tickets are extremely low and even seem to get lower every year.
However, it is good to remember that this ticket has advantages as well as disadvantages:
ADVANTAGES
- It's convenient as these buses follow the usual tourist trail: Saigon-Dalat-Nha Trang-Hoi An-Hue-Hanoi
- During the trip, they also stop at the usual tourist spots
- It's extremely cheap
- Night buses save time and money on accommodation
DISADVANTAGES
- This buses tend to be uncomfortable. Sinh Cafe does have new Daewoo buses with AC, but believe me leg room is not made for my 186 cm (although my girlfriend had no problems with her 165 cm).
- Seats are not reclining
- These buses stop at restaurants for lunch, but these restaurants can be very bad and dirty (though this is not always the case).
- Some roads in Vietnam are awful, so on some trips it is simply impossible to sleep. The trip from Hanoi to Hue is notorious for this, but also Dalat-Nha Trang and Nha Trang-Hoi An. Saigon-Nha Trang on the contrary is quite good.
CONCLUSION
If you care for your comfort and are prepared to spend a bit more, it's best to avoid some trips, especially the Hue-Hanoi trip, and probably also Nha Trang-Hoi An (take a plane or night train from Nha Trang to Danang instead.
Is this your business?