Nine items on the Army's issue list that a lot of riflemen in Vietnam signed for and left in a base camp within the first week: the air mattress, the mosquito net, the rubber poncho, the entrenching tool, the bayonet, the flak jacket, the steel helmet, the M1956 belt and field pack, and the M17 gas mask. Why the list called for each one, why the weight, the heat, the noise or the kind of war made men set it down, what they carried instead, which units made them carry it anyway, and the price paid on the night or in the tunnel where it turned out to matter, with the accounts of the veterans who write in this channel's comments.

Sources:
Shelby L. Stanton, U.S. Army Uniforms of the Vietnam War (Stackpole, 1989), on the M1 helmet and cover, the poncho and liner, the body armor vests, the M1956 load-carrying equipment and the tropical rucksack
Gordon L. Rottman, US Army Infantryman in Vietnam 1965-73 and Vietnam Infantry Tactics (Osprey), on the grunt's field load, what was left in base camp, night positions, bandoleers and the indigenous rucksack
Gordon L. Rottman, Vietnam Airmobile Warfare Tactics and US Army Long-Range Patrol Scout in Vietnam 1965-71 (Osprey), on recon and LRRP dress and the soft hat in place of the helmet
Tom Mangold and John Penycate, The Tunnels of Cu Chi (1985), on CS in tunnel clearing and the protective mask
U.S. Army, FM 21-15, Care and Use of Individual Clothing and Equipment, and FM 21-75, Combat Training of the Individual Soldier and Patrolling (1960s editions), on the individual issue, the entrenching tool and the night position
U.S. Army Center of Military History, Vietnam Studies series (Medical Support of the U.S. Army in Vietnam 1965-1970), on malaria, the mosquito bar, repellent and the chemoprophylaxis program
Accounts from Vietnam veterans and their families in the comments of this channel (used without names)