Activision are looking for voice actors for this year's chapter in the Call of Duty-series and have started to advertise for them - according to VG247. One of the characters, Frank Barnes, is a member of the Studies and Observations Group; an elite unit that was active in Vietnam between 1964-1972.
There has been a rumor about Call of Duty 7 taking place during the Vietnam War before, and this seems to point even further to that conclusion.