As the achievement reads get 100 kills with ALL PRIMARY Vietnam Weapons. The Weapons are as followed:
Assault: M16 A1, AK 47 and the M14
Engineer: M10, PPSh, and the Uzi
Medic: M60, RPK, and the XM22
Recon: M40, SVD and the M21.
If you need to see how many more kills you need for each weapon go to the Vietnam Menu, Weapons and Stats, then just Click on each weapon and it will tell you how many kills you have for each weapon.
As lord verh says it should take about 2 hours for every weapon so it should take about 25- 30 hours of play time to get the 100 kills for each weapon.