You would use the form as follows:
"Property number" "Street"
"City", "State"
"Zip Code"
USA
For example:
1 Market Street
94100
USA
I am unfamiliar with Vietnamese postal operators, so there may be additional items you need to include to route the mail outside of the country, but this is the standard USA address format for posting to the USA from Another Country.
This might give you a starting point, website: Vietnam Veteran's Memorial Wall. They have an information column that address's particulars.
The web address of the Vietnam Graffiti Project is: http://www.vietnamgraffiti.com
The US never invaded Vietnam.
South Vietnam was considered an ally of the US.
The web address of the Fire Base Eagle Vietnam is: http://www.firebaseeagle.org
The web address of the National Vietnam War Museum is: http://www.bunkerflorida.org/
US Military personnel were in Vietnam from 1955 thru 1975.
See: Vietnam
Vietnam
The US was concerned about the development of Communism in Vietnam
We were at war with Vietnam because they disagreed with us about something.
The French were fighting in Vietnam in 1950 not the US.