ok after a little research I found a few sniper rifles used by US, NVA & the Soviet.
US
Winchester Model 70 bolt-action sniper rifles.
M-40 sniper rifle bolt-action. (an older model of the M40A3)
M21 Sniper Weapon System Semi-Auto (or XM21 in test phase).
Springfield M1903A4 bolt-action sniper rifle.
NVA (North Vietnamese Army)
SVD-63 Semi-Auto rifle.
Mosin-Nagant bolt-action rifle.
Mauser Kar98k bolt-action rifle.
Originally posted by another user
(many of the Mausers used by the VPA and the NLF were from rifles captured from the French during the First Indochina War and rifles provided to them by the Soviets as military aid)
Soviet (cold war) <-
this war lasts till late 80's so its hard to tell what guns will be there
Dragunov sniper rifle Semi-Auto. (1963–present)
FN Model 30-11 a Belgian sniper rifle. (1976–1986)
MAS-49 a French-designed semi-automatic. (1951-1979)