It has been said by more than one expat that there are no traffic laws in Vietnam. But although red lights are routinely ignored, driving down the opposite side of the street into oncoming traffic is considered acceptable, and that during rush hour intersections generally resemble one of Dante's circles of hell, it is not entirely accurate to say that there are no rules governing traffic.
Here are the few that I've figured out:
1) The main responsibility of the driver is to avoid hitting anything directly in front of them.
2) The horn is used (almost ceaselessly) to indicate that you are behind someone and are driving faster than they are.
3) Bigger vehicles trump smaller vehicles unless the smaller vehicles have enough maneuverablity and speed to avoid being pasted by the larger vehicle.
4) Generally speaking you never stop unless, a) you choose to stop at a red light, b) death will occur if you don't stop. This is especially true if you are a pedestrian attempting to cross a street. Instead you either speed up or slow down, and if you are driving a vehicle you can change direction. If you are a pedestrian and you choose to change direction in the middle of traffic this is tantamount to playing Russian roulette.
This is what it looks like to drive into oncoming traffic, from the perspective of a mortal (i.e. me) on the back of a motorbike.