Long-distance bus services connect most cities in Vietnam. Most depart early in the morning to accommodate traffic and late afternoon rains, or run overnight.
Public Buses travel between the cities' bus stations. In bigger places, you often have to use local transport to get into the city center from there. Buses are generally in reasonable shape, and you have the chance to interact with locals.
Open Tour buses are run by a multitude of tour companies. They cater especially to tourists, offering ridiculous low rates (Hanoi to HCMC: US$20-25) and door-to-door service to your desired hostel. You can break the journey at any point and continue on a bus of the same company any time later, or simply buy tickets just for the stage you're willing to cover next. Note that if you're not planning to make more than 3-4 stops, it might be cheaper to buy separate tickets as you go (ie Hanoi to Hue can be had for as little as US$5).
Most hotels and guesthouses can book seats for any connection, although you're better to shop around at travel agents, as prices will vary on any given ticket/bus company. Going to the bus company office may net you a commission-free fare, but most major bus operators have fixed pricing policies, which can only be circumvented through a travel agent.
Since tour companies charge very little, they do make commission on their stop-offs which are often at souvenir shops, where you do not have to buy; they always have toilets and drinks and water available for purchase. The estimated time for a bus trip will not be accurate and may be an additional couple of hours sometimes, due to the number of stop offs. Collecting the passengers at the start of the journey can also take quite a while too. Always be at least half an hour early to catch the bus. Try not to drink too much water, as rest stops, especially for overnight buses, may be just somewhere where there are a lot of bushes.
Vietnamese buses are made for Vietnamese people - bigger Westerners will be very uncomfortable, especially on overnight buses. Also, many Vietnamese are not used to riding on long-haul buses, and will sometimes get sick - not very pleasant if you are stuck on an overnight bus with several Vietnamese throwing up behind you.
Even if you are sometimes bus-sick, it is advisable to book a sit at the middle rather than at the front of the bus. First, you will avoid viewing directly the short-sighted risks the driver is taking on the way. Second, you will somewhat escape the loud noise of unceasing honkings (each time the bus passes another vehicle, that is about every 10 seconds).
Although the bus company will usually be happy to collect you at your hotel or guest house, boarding at the company office will guarantee a choice of seats and you'll avoid getting stuck at the back or unable to sit next to your travelling companions. The offices are generally located in or near the tourist area of town, and a short walk might make your trip that much more pleasant.
A scam that you may encounter is that after arriving at your location, the guides will ask you whether you have booked a hotel. Even though you haven't, say that you have and prepare the name of a hotel. If you say you have not booked one, they will charter a taxi for you and probably drop you at a hotel which they can collect commission. If you decide not to stay, things may get a little ugly, as they will demand that you pay the taxi fare, which they may quote as several times the actual fare for a ten minute ride.
