We weren’t just fighting the North Vietnamese but the Soviet Union and Red China. Any use of nukes on our part would have prompted a similar response from North Vietnam’s two major patrons.
In 1965 we had 9,345 strategic warheads while the Soviets only had 929.
We had a 10 to 1 advantage.
It would be foolish for the Soviets to respond because we were the ones who had nuclear superiority.