I suggest you also post to the Ferguson Surname message and the GenForum Vietnam War Message board as well. The more you get it out there the better. http://genforum.genealogy.com/vietnamwar/http://genforum.genealogy.com/ferguson/http://boards.rootsweb.com/surnames.ferguson/mb.ashx
Don could be short for Donald, do you know the state he was born in or enlisted in. What state or city did the sister have her accident. Should be a newspaper account and obituary. There are a couple of companies that have online historical papers for searching. If you don't want to subscribe yourself then see of library has them to use on their computers. Also use subscription genealogy websites. Fold3 has Military databases.
Was you father issued a U.S. Birth Certificate or came as a baby using Don's American Citizenship? He should have been issued a passport and the application (as well as his bc) should be available to get a copy of from the U.S. State Dept. It should have his date of birth, place of birth and even his parents names. Your grandmother's immigration paperwork may have some information to help track down Don.
Also use internet searching his name plus a combination of the words Vietnam, Saigon, Air Force, your father's first name, your grandmother's first name. Search FaceBook, older MySpace, and other social media websites. Could be someone looking for your father if Don had another family later and they found out about earlier family.