VC refers to Viet Cong. The Viet Cong represented the enemy forces in the Vietnam War against the United States' and Southern Vietnamese troops. The Viet Cong was also known as the People's Liberation Armed Forces, representing the Northern Vietnamese forces. Primarily known as a guerrilla force, the Viet Cong possessed some distinct advantages in that they knew the land, had access to better intelligence reports, as well as felt that the primary reason for the war was a defense of their homeland. While the Americans saw the war as a fight for Communism, the Viet Cong saw it as a battle to prevent an American form of imperialism, similar to how they battled off the French form of imperialism.
VC literally stands for the Viet Cong, the North Vietnamese, against whom Perry and the rest of the Americans and South Vietnamese were fighting during the war. Walter Dean Myers uses authentic GI terms throughout the book; so VC is simply one way of referencing the enemy. When the guys talk about "Charlie" that is also the North Vietnamese and is a shortened form of "Victor Charlie" (VC). Victor Charlie comes from the military's phonetic alphabet which designates a specific word for each letter of the alphabet so that letters do not get mixed up over radio communications.