The VRC-12 series was late Vietnam era, but most of the VRC-12 series stuff (such as the AM-2060, RT-524, RT-246, etc) went to Germany, Alaska, Korea, or other Cold-War era hot spots, as a potential war with USSR was a higher priority than was Vietnam. Most of the radio gear used in Vietnam was older 1950s and early 1960s (pre-transistor) tube units, such as the RT-66/67/68 transceivers, the older GRC-19 radio set (made up of the venerable Collins R-392 receiver and the T-195 transmitter), or the RT-70 compact vehicle transceiver.
Same story goes for other late-Vietnam era equipment. I have a M725 ambulance, and although many of the M-715 series trucks were made during Vietnam, these trucks mostly went to Cold War hot spots, and Vietnam area had the M-37 power wagons, which were older line and worked just fine in that campaign....
