Vietnam has a pretty good public transport system compared to other parts of Southeast Asia including planes, trains, and buses that will take you almost anywhere you want to go in-country.
Keep in mind however that Vietnam is relatively large, so even though getting around may seem easy, if you try to do everything you will spend most of the time travelling and not seeing much of the country.
Travel by bus is cheap in Vietnam, although try and avoid travelling over public holidays, particularly Tet, when it will be impossible to get a ticket to anywhere unless you have booked in advance.
Also, be aware that Vietnam night buses are anything like a comfortable ride and getting any decent sleep is an illusion.
Traveling by train is also cheap and often a good alternative for the (night)bus.