Steam is not paying taxes in Vietnam, and they have no official employee in Vietnam, that serves as a contact person. The latter is required by law. E.g. Shops need to have a shield on their front, giving contact information, and communicate clearly who's behind this shop (private person or company).
Doesn't seems to be an official ban. My friends in Vietnam are still able to access Steam just an hour ago. This is most likely a situation like those manga websites black listed by Comcast or AT&T DNS.