One huge difference between Western and Vietnamese cultures is the perception and treatment of personal space. In the West, it seems everyone has a "personal space bubble (PSB)" around them at all times. Westerners monitor their PSBs closely. They are acutely aware of who is in their PSB or who may be moving into their PSB. Usually on the street, two strangers will move around each other in such a way so as their PSBs never touch each other. When stepping onto the street or merging in traffic, Westerners generally check to see if their action will interfere with someone else's PSB. If so, the person already on the street or on the highway is given right-of-way so that the PSBs do not touch. We execute this action automatically and without thought.
There are no PSBs in Vietnam. None. Zero. Bupkus.
No one maneuvers carefully out of the way, except to avoid a collision on the sidewalk or on the street. When I'm walking on the sidewalk it's a common occurance for a guy to zip up and park his motorbike right in front of me with zero disregard for my PSB. He didn't even see me. If he had, he wouldn't have cared.
People merge into traffic from side streets all the time without looking. They brake, weave and rubberneck with little if any regard for the effect their behavior has on traffic around them.
The other week I was in speaking with a cluster of people in an impromptu meeting with a partner company. One young staffer quite literally elbowed me out of the way so she could walk past. I met eyes with another Westerner and we shared a chuckle.
Westerners generally are more aware of their surroundings and the effect their motions or actions have on others. The way Vietnamese sometimes behave in crowds is considered quite rude in the West. This is the reason that such behavior really grates on some Western expats I know. They just hate it, and react to it by painting the Vietnamese with a broad brush of "They are all rude and inconsiderate." I know what they mean when they say it grates on them. There are times when it grates on me too.
But I'm adapting. I only get upset now at the most foolhardy or stupid cutoffs I get on the road. Others, I let slide by. It's not rude by local standards, and I subscribe to the "When in Rome..." philosophy. Besides, now I do my fair share of cutting others off in the perpetual game of chicken that is driving in Vietnam. I've also learned that, when crossing the street, do not let oncoming traffic see you looking at them. If they see you look, they will expect you to yield to them. If you don't look (or look out of your peripheral vision like I do), you put all the responsibility on them for a collision. It sounds crazy, but it works.
The best explanation I can think of regarding why the Vietnamese are like this is that there are a whole lot of people in Vietnam. That, and a culture that includes frequent touching within families and between friends. People simply don't have the same feelings about personal space that Westerners do. There's no sense in getting worked up about it. That's just the way it is here, and it's not rude by local standards. It's a good litmus test -- Westerners who have a difficult time adjusting to it will have a hard time to adjusting to lots of other things here in Vietnam.