The flak jacket would have two different models in use during the Vietnam War. The first model, the M1952A, would be surplus from use during the Korean War. The M69 would be developed during the war, and would have a few minor changes. The purpose of both flak jackets was to catch the "flak" from explosives like grenades and landmines, but wasn't designed to stop bullets, and so soldiers were still vulnerable to enemy fire.
The M1952A Flak Vest:
The M1952A Flak Vest was popular among the US Army. It was made from ballistic nylon, and weighed in at six pounds. The vest was worn over the utility shirt, and was put on like a jacket. Before the use of explosives became common among the NVA and Viet-Cong, there was a general sense of dislike for the vest. It was heavy and felt like an unnecessary burden, lacking the ability to stop the more common bullet from an AK-47. The M1952A had straps on the shoulders, and no protection for the neck, as can be seen to the left.
The M69 Armored Vest:
The M69 Armored Vest was the modified and upgraded version of the M1952A. It offered neck protection, shoulder straps which were less likely to be caught on the environment, and was reinforced. As a result, the new M69 weighed eight-point-five pounds. The M69 was also disliked, except that this model had a more annoying neck protection area that would bump into the helmet, and it was heavier. US infantry troops would typically not wear their M69s, except for the US Marines, for whom it was mandatory.
M1952A Flak Vest
Arques, Antonio. "Body Armor." Grunt: A Pictorial Report on the US Infantry's
Gear and Life during the Vietnam War 1965-1975, by Arques, Madrid, Andrea
Press, 2014, pp. 241-48.
M1952A Flak Vest on a US Soldier in Vietnam
M69 Flak Vest on a US Soldier in Vietnam
M69 Flak Vest