Why are the North Vietnamese Army (NVA) referred to as "Viet Cong"?

"Viet Cong" is the term used by the presidential administration of South Vietnam's Ngo Dinh Diem to broadly classify the Viet Minh forces as pro-communist in a very derogatory way. In fact, the term connotes "little commies", which is a dismissive and short-hand way of objectifying the enemy for the ARVN.