For the first question. What caused Nixon to resign, and the largest thing, was the Watergate-Scandal.
For the second question. The Vietcong followed the guerrilla method of war and had hidden bases and networks of tunnels. They generally refused to fight large scale battles in the open, instead opting for hit and run attacks and raids.
Lastly, Nixon's China visit was of particular significance because it marked the beginning of a process of normalization in etc ChaCha On!
