Series 641 of the military payment certificates was used in Vietnam exclusively (there were other series for other areas of the world) from 31 August, 1965 through 21 October, 1968.
The MPCs were used so that soldiers didn't have to carry US dollars; to prevent use by unauthorized people (they were given to US military personnel to be used by US military personnel) and to prevent a black market (they were withdrawn from circulation and thus rendered worthless without notice.)
Series 661 replaced 641 on 21 October 1968 and was withdrawn on 11 August 1969. Here is the 5-, 10- and 50-cent denomination note.
The series 661 1 dollar note.
As a contrast, here is the 1 dollar from roughly the same period issued to US troops in Korea, Japan, Libya and Cyprus.
Series 681 replaced 661 in Vietnam and was in use from 11 August 1969 to 7 October 1970.
Here is the 25 cent note from that series. It had a more 'military' theme to it picturing submarines, soldiers, bombers and tanks.
