58,000 Americans were killed in Vietnam and 153,000 wounded. Using nuclear weapons in 1964 would have ended the conflict and saved American lives.
We will never know what the effect of going nuclear on Vietnam would have had on the Cold War and the world, but I doubt that it would have been pretty, it probably would have led to a massive acceleration and hardening of the global war footing and a more desperate search for a way to survive once an unstable, renegade America was revealed as a threat to everyone, and every city on the planet.
By the way, we had already lost 400 men before 1965.
Nukes on an enemy the United States had already defeated would have been less necessary than taking the enemy within the US who really lost it for us, and who are very much still here, and even elected to be in charge. Should we nuke them too?