Yep, the VC would come down from the North and infiltrate the south, many time in slums or countryside and ambush our troopes and arvn.
I can tell you my Montagnards killed VC like flies on the local security OPS we ran. We mainly fought NVA and not VC. A 32 man unit(30 yards and 2 SF NCO’s kicked entire VC companies out of their bunkers as the Montagnards knew the jungle, highlands, etc, better then any Viet whether NVA, VC, or sad ass ARVN.