From my personal experience, Vietnam is more problematic than Thailand in that regard. Most of the scams I experienced in Vietnam were minor like not giving me the exact change, for little sums but I had also a very bad experience in Halong Bay, the popular tourist destination in the north, an experience that ruined my visit to a wonderful site.
You can read about this later, on this page. Leaving your passport at the reception may sound, at first, like a good idea to avoid theft or losses but it is not.
When you are in a foreign country and someone hold your passport, you are at his mercy. You can not leave the country without your passport. If the hotel decides to make a scam on you, like for example claiming that you need to pay extra for your room because it is special in some way, then you will be under a lot of pressure, when they hold your passport. The bad experience that I had in Halong Bay , was basically possible because they had my passport.
The possible solution to this : Pay the hotel in advance and then you can keep your passport. By the way : You need to show your passport in order to change currency, in town.
So, here is another reason to keep your passport with you. In your own country, you are certainly on your guard to avoid scams and thefts, so in Vietnam you should be even more alert. For example, you should double check the change you receive when you pay for something.
