i do not know if the same procedure will be applied in Vietnam. I am also on family visa and I came from the Philippines. I sent my passport together with a copy of the visa issued by the ministry (that is in Arabic) through DHL back home addressed to the embassy since they do not allow personal appearances of applicants. The Qatar embassy, if there is any in Vietnam will translate it to English. It would appear to be an invitation letter or something that has to be shown to the immigration officer at the airport before checking in. Hope this helps. For more info, it will be better for your wife to make the inquiries in her home country.