Unified Vietnam, 50.Anniversary of the Battle of Dien Bien Phu, May 4th, 2004, Michel Nr. 3328-3330, Block 139; Two stamps in nominals of 800D and 3,000 were issued in perforation 13.25. There was also a souvenir-sheet with a nominal of 8,000D that was perforated 13.5:13.75.
Here is the mint set
Interestingly, France also issued stamp to remember the defenders at Dien Bien Phu on May 10th, 2004 with a €0.50 nominal and in perforation 13. Here is a mint block of six of the French stamp.
Multiple franking of the French Dien Bien Phu stamp paying an overall postage of €2 on an international express mail letters ent from Talmont Saint Hilaire to Ho Chi Minh City in September of 2004. Kai Thac Van Nguyen arrival cancel on the reverse.
Registration Nr. 108310