In the show, Nguyen is pronounced "New-Yen". The correct pronunciation of this Vietnamese name is "Win". Furthermore, Nguyen is usually a surname, not a given name. Also, if the son is in high school which means he could be anywhere from 14 to 17 years old (the actual actor was 15 at the time). The US first started sending troops to Vietnam around 1965, so the father would have had to leave around 1967, come home and get married and have his son within two years of returning home. However, the war was in full swing around 1967 and it is unlikely any soldier would have been released at that time. Furthermore, the actress who played Nguyen was 28 at the time, born well before any US troops were sent to Vietnam.
The actress playing Nguyen is not Vietnamese, but rather of Japanese origin born in Hawaii.