Later, when Nixon became president, the situation changed for the better, but it was a slog. Nixon had promised the South Vietnamese premier better terms than what LBJ was offering, but, by the time he came into office, the situation had gotten so bad that it was all that he could do just to guarantee South Vietnam's existence.
Nevertheless, the situation stabilized. The North Vietnamese army was pushed out of South Vietnam. Nixon got the North to sign a cease-fire agreement, which the North (being perfidious communists) violated.
And the South Vietnamese army got good enough that, after Nixon had pulled American ground troops out of the area, South Vietnam was able to defend itself from North Vietnamese aggression, for a while.
In 1972, North Vietnam sent 150,000 troops—along with columns of tanks from the Soviet Union—to invade South Vietnam. The latter had American air support, but only its own troops on the ground.
Result: Less than a third of North Vietnam's invasion force made it back north. Roughly 650 Americans were KIA in a campaign as big as the Battle of Kharkov. The Army of the Republic of Vietnam had won, big time.
Then, Watergate happened.
In 1975, North Vietnam—violating the cease-fire agreement AGAIN—launched another invasion of South Vietnam. The Soviet Union continued to support North Vietnam. The Congress of the United States of America pulled all support from South Vietnam.
Result: Yet another workers' paradise blighting the history of the twentieth century. And the kind of gloating you can see in this discussion thread.