Your Vietnamese Fiancee needs permission from the US government to allow her or him to enter the USA. This is called a K1 Fiancee visa. Click here for description of the process in Vietnamese
Fiance visa applications are held only briefly at NVC, usually a few weeks, just long enough for NVC to assign a new case number and forward your file via diplomatic pouch to the consulate in Ho Chi Minh City. Case numbers assigned to Ho Chi Minh City begin with HCM
Once the case arrives at Ho Chi Minh City, the visa interview can usually be conducted in about 1 to 2 months later.
And do be aware, that even though there are 2 locations of US consulates in Vietnam, in both Hanoi and Ho Chi Minh City, the US State Department has decided to concentrate all consular staff involved in processing, vetting and interviewing of Fiance visa applicants, in just ONE place, the consulate in Ho Chi Minh City. So regardless of where your Fiance lives in Vietnam, her or his interview will only take place at the US consulate located in Ho Chi Minh City.
By Fred Wahl