What advantage did the vietcong have over American troops?

They had many advantages over American troops. They were intimately accustomed to the terrain and local wildlife/ fauna, they didn't have to fight playing by international rules, they had already established a large network of tunnels and supply routes before the Americans arrived, they blended in with the local peoples, they knew the language and the culture, they used terrorist tactics, they had information networks that covered the entire country and had the sympathy of the public, they were immune to many local diseases that the Americans were not, they didn't rely on questionable and fickle technology and were unified in purpose.