Driving in Vietnam is a unique experience, especially to those that are used to driving in western countries. Expect roads extremely busy, such as the A1 highway that runs from Ho Chi Minh City to Hanoi. To make things easier and your commuting safer, there is a largely applied system that you should be aware of.
To begin with, you should avoid the A1 at all costs. Only the section that takes you from Hue to Da Nang is recommended as it will lead you through the infamous Hai Van Pass. Admittedly, driving in the city will be a challenge at first, but once you get down to driving, it will actually become very fun indeed!
Vietnam is the country where the sound of the horn is heard ridiculously often. It is the way locals are shouting to one another to pull over for a number of reasons. So, get ready to use it like never before as you will meet many drivers that change lines without even looking first. That aside, it is incredibly stressful to see children roadways that don't mind darting out into the road without even looking before. Therefore, if anyone seems like they are going to do something like that, make sure they get their warning. Go hard on the horn to make your presence known.
Never expect that other vehicles will move out of the way for you, even if they know you are there. There is an unwritten road law in Vietnam according to which the larger vehicle always has the right of way. This means that if you see a bus passing a truck and heading towards you in the opposite lane, leaning on its horn, you should move aside else it WILL crush on you and you will be talking to the pavement. This is why you will see everybody else driving smaller vehicles pulling onto the shoulder. Also, larger vehicles are very likely to take the lane so stay alert, keep your head up, and your eyes open when driving on busy roadways.
When it comes to traffic lights, Vietnamese drivers generally adhere to the universally applied rules. This means that they will obey red lights when in major cities and busy intersections. However, there is a dramatic change of scenery (and road attitude) when you get into smaller towns or the countryside. A local driver will have absolutely no problem flying right through a red light if they don't identify any immediate danger. So, it is not uncommon to see vehicles running lights at intersections. And, if it is a quiet one, it is probably OK. But, it is a totally wrong (and dangerous) idea to disrespect traffic safety rules in the big cities. Just wait for the green light and go ahead.
Finally, you will come across locals that don't use their turn signals. In this case, if you see a hand waving or pointing into a particular direction, be prepared as they are about to turn! All in all, it is strongly advised you don't forget your western driver training, regardless of the traffic behaviors you will see here.