There are many ways of moving around in this country: rail, bus, car, motorbike, hired driver, plane, hydrofoil, and boat. My preferred method is airplane but due to the high cost of air travel, I usually go with car & driver. For long distances car and bus are much more comfortable than motorbikes. Hired drivers are a good way to get around Vietnam. The cost is relatively cheap and it is more comfortable than a bus. You also have more freedom to plan your itinerary as you want. Hydrofoil is a great way to get from Saigon to Vung Tau beach (around 90 minutes away). This is an excellent day trip and round trip is around 15usd per person. The many rivers of Vietnam can be explored on slower fishing boats or on high speed boats used for long distances. With almost 18,000km of waterways to explore in Vietnam, a boating trip should not be missed.
By far the most popular method of transportation in Vietnam is the motorbike. Motorbikes here are cheap, easy to maintain, and fuel efficient (~100mpg/160kpg). If you have a problem with your motorbike such as a flat, engine problems, electrical, etc. you can find a mechanic within 1km from you wherever you are in the cities and within 20km on the highway. In the city, traveling by motorbike is generally much faster than by car because the small size of the bikes allow them to bypass traffic jams with ease. However, motorbikes are extremely dangerous: the roads are crowded and in poor repair, people often ignore traffic rules, large trucks drive very fast and often in close proximity to the motorbikes, and the helmets here are poor and offer little protection. Even though dangerous, driving a motorbike in Vietnam is extremely fun. Below is a sample of the traffic in HCMC to give an idea of the number of motorbikes here.
My recent purchase here was a new underbone- the 135cc Yamaha Exciter (known globally as the t135). This is about the closest I can get here to a large displacement motorcycle without spending a fortune (motorcycles with engines over 149cc are taxed 100% making them twice the cost of a bike in the states...). My motorbike has a clutch with gearing set up the same way as a conventional motorcycle- 1st gear is all the way down, neutral above it, 2nd above neutral and so on. It is a four speed so the top speed is in the neighborhood of 140kph which is plenty fast in Vietnam.
If you come to visit Vietnam I would recommend taking one of the motorcycle taxis around the city. They are called "xe om" and are about half the price of a regular taxi (note that you should negotiate your fair before you go). The xe om is a good way to see the city and get a feel for driving a motorbike in Vietnam. If you are feeling a bit more adventurous you can rent motorbikes here for the day, week, or month and the rates are very reasonable. Be aware though that national drivers licenses and international drivers licenses are not accepted in Vietnam and one should get a Vietnamese drivers license before driving here.