March 29th is Vietnam Veterans Day, apparently because on that day in 1973 US forces withdrew from the war. How come March 29th 2016 is the 50th anniversary according to this article and several others?
On March 29th, 2012, President Obama released a proclamation naming March 29th to be Vietnam Veteran's Day. The last US forces in South Vietnam left the country on that date in 1973.
Now, to actually answer your question: It doesn't. In fact, Obama's proclamation names 2012 as being the 50th anniversary of the Vietnam War. 1966 was not the start of the Vietnam War and if you're like me and count the beginning of the American participation in the Vietnam War with actual ground troops (that is, 1965) then last year would have been the 50th anniversary.
It's strange as to why 2016 has been dubbed as being the 50th anniversary and I can't find a logical answer to give you. I will assume it's an error.