The last name, in Vietnam, is there, but just isn’t that important. And when it’s not that important, you might as well change it if a new last name might help you in some way. This may or may not be a continuation of the way names were used before the Chinese came—we really don’t know—but ever since, Vietnamese people have tended to take on the last name of whoever was in power at the time. It was seen as a way to show loyalty, a notion which required the relatively frequent changing of names with the succession of rulers. After all, you wouldn’t want to be sporting the last name of the previous emperor.