Vietnam is much farther from Washington than Kiev is from Moscow. We know exactly how the USA reacted when the USSR sent serious hardware to Cuba.
Namely:
https://en.wikipedia.org/wiki/PGM-19_Jupiter to
https://en.wikipedia.org/wiki/Izmir_Air_Station in
https://en.wikipedia.org/wiki/%C4%B0zmir
Now look at a map and compare the distance to the former territory of the Soviet Union and associates, and compare that with the distance of Cuba to the CONUS.
Whining pinschers...
We're past that point since a long time ago.