why were the vietcong important in the vietnam war?

a) The term Viet Cong is derogatory shortened slang for "Viet Nam Cong San", meaning Vietnamese Communist, something that less than 4% of the NLF were members of.
b) The "National Liberation Front of Southern Vietnam" (NLF) was created at a conference just outside of Saigon in December 1959 by a mixture of Nationalists of vatious political opinions, Lawyers, Buddhist Monks, Trade Union officials, communists etc. Their first combat action was in early 1960 when they attacked an ARVN police station taking all the US weapons inside. Before 1960 the disssidents of all sorts were just protesting, not fighting.
c) The Second Indochina (Vietnam) War started in May-June 1954 when the US, under Eisenhower as President, Nixon as VP and John Foster Dallas as Secretary of State, violated the "1954 Geneva Agreements on Indochina".
d) The actions of the illegal South Vietnamese government and its soldiers, along with the actions of the US and their military caused many non-political Vietnamese to turrn against both the South Vietnamese government and the US and join the NLF either as part-time or full-times fighters.
e) The NLF did all the fighting in the south between 1960 and late 1965, and only then did reinforcements by the "People's Army of Vietnam" (PAVN) start to arrive in the south starting June-July 1965 with the first major fight between the US and the PAVN in November 1965 at Ia Trang Valley , of "we were soldiers once, and young", book and film fame.
f) Yes, the NLF was heavily dessimated during TET in 1968 but recovered within 15-18 months, but not to its previous levels.
g) The NLF have the Vietnamese people in the south an idea of a potentially better future, something the South Vietnamese government and its allies could not.

The Viet Cong were a Communist dominated rebel movement in South Vietnam which was supported by North Vietnam. The Viet Cong were opposed to the government of South Vietnam, which was reciveing supported from the United States. For most of the war they were the primary combattants on the communist side.