I have a theory about Korea, I can't prove it but it fits the facts and bears consideration.

The US responded to N Korean aggression by invading under the UN flag and with token military support by other UN members. It probably would have done so anyway, but going in with UN support lent additional legitimacy to the operation. The Soviet Union could have denied the US this support by simply vetoing the Security Council resolution that authorized military action. The US would have had to scramble around and do it solo, and it would have taken time and cost us politically. But instead, the Russian UN rep walked out of the Security Council vote in a huff of protest, allowing the US to get its way

The conventional wisdom is that the USSR misjudged the US resolve and just bungled the diplomacy, but I have another suggestion. I believe the Soviets wanted war between China and the US, both were rivals, one to leadership of the Communist movement, the other to world leadership. It was in its interest that war break out between the two, and it was likely to spread outside Korea once things got rolling. I think both China and the US got suckered into this one.

As for the differences between Vietnam and Korea, in the former it was the US and France who did everything they could to stop unification and free elections because we knew we would lose. In retrospect, I think we should have been in both Viet Nam and Korea, I just think that in the case of Vietnam, we backed the wrong side.