Unified Vietnam, 30. Anniversary of the Battle of Dien Bien Phu, May 5th, 1984, Michel Nr. M42; A single stamp with no nominal value was issued in perforation 11.0.
Here is the mint stamp
This stamps not officially released imperforate, however a number of imperforate trial proofs have come to market. Here is an imperforate vertical pair from the top margin of the sheet.
Official First Day Cover
Single franking of the stamp on a local letters ent from Hanoi to HCM City in November of 1986.
Because the stamp did not carry a nominal value and was handed out free of charge as a social cause the free franks were not valid for international mail. It appears however, that not all postal clerks were aware of this fact or simply did not care. Here is a registered air mail letter from September of 2008 that carries a block of four of the free frank together with the Dien Bien Phu stamp/souvenir-sheet set from 1984 paying an overall postage of 34,300. That appears to be the required postage (as per the annotation on front) so the free franks were ignored and valued at zero but cancelled nevertheless.
Registration Nr. 103625