The "Open Tour" is a daily bus service. You can travel trough whole Vietnam from north to south or vice versa for as cheap as 29 US$. However you should make sure to book not to cheap to get into a bigger and enjoyable bus.
Moreover, on the journey, at your demand and interest, the traveller could stop by and visit any famous destinations nation wide.
e.g. Hanoi - Ninh binh - Vinh - Quang binh - Hue - Danang - Hoi an - Nhatrang - Dalat - Ho Chi Minh City or vice versa