Vietnam is in Vietnam, it has not moved. It is a physical place.
If you mean to ask Why have Vietnamese people come to the United States, please click on that.
See Vietnam Time Line
The last U.S. forces left South Vietnam in 1973. The US Embassy was evacuated and the last advisers pulled out in April 1975.
Which containment does Vietnam come from
The Republic of South Vietnam (RVN) controlled themselves; hence the word "Republic." Note: The US/allies were invited to come (asked to come) by the RVN.
North Vietnam conquered South Vietnam.
The US never invaded Vietnam.
South Vietnam was considered an ally of the US.
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.