museum of ethnology
To learn about the various ethnic groups in Vietnam (and there are 56 of them), the Museum of Ethnology is a great place to spend an afternoon in Hanoi.
The circular main building is dedicated to exhibitions of the ethnic groups in Vietnam, and a separate contemporary building houses stuff from the rest of Southeast Asia. Behind both buildings are actual houses from ethnic groups which were relocated to this museum, and visitors can explore the inside of these homes to see how people live. It’s an awesome museum and well worth a visit.