Why doesn't the US send airplanes to Ukraine, when the Soviets sent MiGs to North Vietnam?

During the Vietnam War, the Soviets sent hundreds of MiGs to North Vietnam. Now, Americans say they cannot send airplanes to Ukraine lest it lead to escalation.

Why wasn't the Soviet assistance to North Vietnam during the Vietnam War a form of escalation? Why didn't it start the 3rd World War, say, via an American attack on the USSR as revenge for the latter's direct involvement in the Vietnam War?