Long-distance bus services connect most cities in Vietnam and tickets can be purchased at the Ben Thanh Bus Station. However, Vietnam is a very large country with a coastline that runs for 3,444 kilometers. It is advisable to break your bus journey to destinations within six hours' drive.
A multitude of tour companies run multiple bus-stop journeys called Open Tours that run up and down the country. For a ridiculously low price, you can choose to stop in a destination and stay for as long as you like. When you decide to get back on the bus, go to their office the day before you plan to leave.
But there is a catch to this convenience and low price. You may be brought to a well-established network of hotels at each stop and pressured to stay at each. If so, hold your ground and insist on being dropped off at your destination.