It was illegal to have American dollars in Vietnam, so we were paid in MPCs, Military Payment Certificates MPC's, In October 1968 the series changed from Series 641 to 661. This was done in an attempt to render useless the money hoarded by black marketeers.
Although the local money was universally called "piasters", this was a
holdover from the French Colonial days. As you can see, all of the Vietnamese money was calculated in "dong". In the light blank area on each bill you can see, if you hold it up to the light, a watermark. Unfortunately it doesn't show up on the scans.