Nice thread. Yes, most people in Vietnam are friendly and accommodating to foreign tourists, particularly in the smaller cities and villages. Its typical for people and children to say hello to you and to try to start a conversation or to get invited to some coffee or food by people in coffee shops and restaurants.
There are black sheep also, tricky people, particularly taxi drivers and tour agencies / tour guides (they'll sell you one thing, but give one of a lesser value).
Try to use the green taxis, that's the best company and they have better control of the drivers.
Crossing a busy street in the big cities can be a hazard, keep in mind that most drivers in Vietnam feel that they have the priority and expect that you'll get out of their way. If crossing a very busy street, try to walk next to a Vietnamese. When crossing the street is very important not to do unexpected, sudden moves. The driver will calculate what you are doing, but if you move in an unexpected way, then you put yourself at risk.
Also, when crossing the street, be very mindful of the motorcycles that are TURNING into that street, I feel those are the most dangerous, you can be crossing on a green light, which means that you have the priority, but drivers may just turn onto the street and act like if you and the green light don't exist at all.
When taking buses for long distance travel, use the sleeper busses, they are really good, but ALWAYS USE A HIGH QUALITY BUS COMPANY such as Sinh Cafe, FUTA, etc.
In the big cities, particularly in HCMC, be careful with your phone / hand bag. Guys in a motorcycle can pass right next to you and snatch it.
In some tourist areas in the big cities, street sellers may come to you and push you to buy something, don't give in to them, you don't have to buy, don't believe their stories, but if you buy something, better bargain hard because almost for sure they are overcharging you big time.
In most shops for souvenirs / gifts etc, you are expected to bargain. Don't believe anything they tell you, like: this is from America, this is very high quality, etc. They'll say anything to make a sale.
When checking in in a small hotel, make sure about the price in writing, get a slip of the room rate, make sure about what you are supposed to get so that there is no surprises at the time of checking out,
Renting a regular scooter motorcycle should run between 100,000 and 150,000 vnd per day / $5 to $7 usd (less if weekly rate). 1 to 1.5 million vnd per monthly rate $45 to $70).
i would not advise to drive in HCMC or Hanoi due to intense traffic. Other cities are ok.
Beware of cheap coffee in the small places / street coffee shops. The coffee is often mixed with soybeans, colorants, etc. Its actually partial or fully fake coffee, you will not feel the effect of caffeine because it has none.
I'll think of more things and post later.
Edited: I'll keep coming back to add more things.
