Duck and Cover

Short Story by Don D'Ammassa

A low-level military intelligence guy in Vietnam begins to investigate one of his fellow soldiers who just isn't quite right.

This is almost entirely a Vietnam war story with all the SF content implied rather than outright stated. And it is creepy. Also very well written, but not a whole lot happens, so just sort of a spooky war story. 3.5 out of every 5 soldiers are named Elmer Colby.