Collectivism
In general, the Vietnamese are a collectivists.
The individual is seen as secondary to the group - whether the family, school or company.
As a result there are strict guidelines for social interaction that are designed to protect a group's face
Hierarchy
As with most group-orientated societies there are also hierarchical structures.
In Vietnam these are very much based upon age and status.
This derives from Confucianism, which emphasizes social order. Everyone is seen as having a distinct place and role within the hierarchical structure, be it the family or workplace.
An obvious example is seen in social situations where the oldest person in a group is greeted or served first.
Within the family the head would be responsible for making decisions and approving marriages.