So, within a family you will see different last names.
But the children will carry the father's last name.
Polygyny was widespread in both northern and central Vietnam, as was the taking of concubines.
Besides the so-called wife of the first rank, a household sometimes included a second and third wife and their children.
Women are generally expected to be married at a relatively young age and to have children frequently thereafter.
Out-of-wedlock children are not generally approved; their birth is severely censored: in a family-centered society as Vietnam is, the place of such children is quite difficult.
Polygamy is illegal but some men have a second, unofficial wife.
When Vietnamese marry, the woman doesn’t change her family names.
In the past Vietnamese marriages were arranged through matrimonial agents (mai-dongs) who brought the two families together and arranged the question of the wedding portion (bride price).
Interestingly, the woman did not bring any marriage portion, and it was the groom who paid for the wedding presents, brought to the common lot his fortune of rice fields and cattle, and often had to pay money to the wife’s family.
Because the individual is less important than the family, it is expected that the family will have a major voice in the selection of wives and husbands of their children.
This is often done through a "go-between" (male or female) to save "face" in case it is deemed best to break off bargaining.