I was assigned to the Joint Casualty Resolution Center (JCRC) in 1973-1974 and spent some time back in RVN before the Commies took it over in April 75. There was an Air Force officer assigned to the Saigon office named Paul Mather who wrote a very complete account of the efforts to recover/resolve MIA's and dead body not recovered (Dead-BNR). The title of the book is "M. I. A.: Accounting for the Missing in Southeast Asia." It is out of print but available through Amazon dot com. I was the Army Crash Site Investigator and my memory is a little hazy, but I remember that to declare an MIA as dead required Service Secretary approval and had to be requested by the family. Once delared dead the pay and allowances ceased and the life insurance was paid, so there were some "practical" reasons for not requesting it.
I participated in some of the recoveries and it was pretty rough work at times. We had to avoid the bad guy areas and not carry weapons. A good friend of mine was killed (murdered) on Dec. 15, 1973 in an ambush south of Saigon. Things have changed a great deal since relations are better between the US and Vietnam. Recoveries are accomplished with more cooperation and we will likely see more like this one. One big problem is that remains are perishable and time is not on our side. I was in the Army for 20+ years and that assignment to JCRC was the most personally rewarding of all. During my time there we recovered about 25 sets of remains.