Richard Nixon famously sabotaged the Vietnam peace talks during the 1968, promising the South Vietnamese government a better deal once he was elected. Let's say Nixon hadn't done that (the reasons he doesn't do it don't really matter). Would Johnson have been able to achieve peace in Vietnam?
I doubt it. The Tet Offensive applied
Sun Tzu tactics:
"Therefore, the best warfare strategy is to attack the enemy's plans, next is to attack alliances, next is to attack the army, and the worst is to attack a walled city." - (03.03 Art of War)
The Vietcong scored a PR victory.
"If the army is involved in a long campaign, the nation's resources will not suffice.
When weapons are blunted and ferocity diminished, strength exhausted and resources depleted, the neighboring rulers will take advantage of these complications." (02.05-02.06 Art of War)
They knew if they dragged the war on long enough they would win and that is exactly what they did.