Rural Vietnam has a large number of people from other ethnic groups. They typically have their own history, customs and language. Some understand the Vietnamese language as well but others do not. While it may not be characteristic of all of them, they can often be distinguished by their darker skin colour.
These "minority people", as they are often generically called, are typically not as well off as other Vietnamese people. Many lead a very simple life in rural or mountain areas, chopping wood to trade for rice to eat or hiring themselves out for whatever tasks they are able to find. This man was tending someone else's cattle. They are often shy of even Vietnamese folk let alone a foreigner like me. At another location the same day as this photograph was taken, an entire family more or less hid away from my wife's family. We had stopped where these people lived under a simple tarpaulin shelter by a stream. They stayed away out of sight and only returned after we left.
A number of these people had been recently relocated by the Vietnamese government to this area. The government wanted to develop their former home area for other purposes.
The expression on this man's face may appear to be annoyance at me taking his photograph, but it is not. It is simply a part of his character.