Vietnam nukes = 0; USA nukes = 3,500. No usage?
The other scenario reduces to that one: a first strike takes out the other power’s nukes.
Too risky. Impossible to do a 100% first kill. Nuke subs also makes this impossible.
With regard to the latter, don’t forget that neither Putin nor Xi would be bothered by losing 20% of their populations if it meant the total destruction of the US.
Surrounded by allies in 1945 Hitler chose not to use gas, the equivalent of nukes in 1945.
Vietnam: not worth the blowback. Should have been used against Mao during Korea.
First strike: depends on the sub-tracking tech. If you have enough of your own submarines or some other way to track them, it’s possible
Gas isn’t a strategic weapon, although I’m fairly certain if Hitler had been able to gas London or Moscow effectively, he would have.