It would have made very little difference. The US would have intervened in Korea with or without the Security Council's approval. Moreover, it would eventually get UN sanction--from the General Assembly (under a "uniting for peace" resolution of the kind that was eventually used in OTL)--but that would not be essential.
Moreover, a Soviet veto would not even have delayed US intervention, as I once noted in soc.history.what-if:
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I don't think it would have delayed it at all. To quote my old college history textbook: Already on June 25, 1950 at a top-level conference presided over by Truman at Blair House "[E]veryone agreed that the southward drive must be stopped, even if the United States had to fight alone." Richard Leopold, *The Growth of American Foreign Policy*, p. 682. On June 26, when it was apparent the North Koreans were driving ahead unchecked, another conference decided to go beyond the Security Council's request for aid in effecting a cease-fire and use US air and sea forces to cover and support South Korean forces. This was at a time when the US could not be sure whether the USSR would return to the Security Council. As it turned out, on June 27 the Security Council did pass a resolution urging members to provide military support to the ROK--but there was no assurance *in advance* that the USSR would not return and veto it.
Truman's decision on the 26th *anticipated* action by *either* the UN or Congress. There is no reason to think he would have made a different decision--or would have delayed--if the Soviets had been exercising their veto. Truman did not finally decide to use ground troops until June 30, when it was apparent that the navy and air force could not halt the North Korean advance. Again, I don't see any reason to think it would have taken him any more time had there been a Soviet veto in the Security Council.
You have to remember that the "lessons"--however simplified some might think them--of "appeasement" in the 1930s--were very much on Truman's mind. (And on top of that, he was being hounded for the "loss" of China, for "softness"
on communism, etc.) In view of all these things, to suppose that a Soviet UN veto would have altered fundamentally his response to the North Korean aggression seems very implausible to me.
https://groups.google.com/d/msg/soc.history.what-if/rfe72FCDn4E/luf3M8zXVfoJ
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Elsewhere, I wrote:
An emergency General Assembly meeting could be called very quickly, but it's not as though Truman would think his hands were tied in the meantime, anyway. He would presumably rely on Article 51 of the UN Charter [1]:
"Article 51 provides for the right of countries to engage in self-defence, including collective self-defence, against an armed attack. 'Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.'
"This article has been cited by the United States as support for the legality of the Vietnam War. According to that argument, 'although South Vietnam is not an independent sovereign State or a member of the United Nations, it nevertheless enjoys the right of self-defense, and the United States is entitled to participate in its collective defense'..."
http://en.wikipedia.org/wiki/Chapter_VII_of_the_United_Nations_Charter
A General Assembly meeting would simply be useful in providing retrospective justification for the intervention Truman had already authorized. (Incidentally, Truman did not grant MacArthur's request to transfer two whole divisions from Japan to Korea until June 30. What led to this delay was not the lack of UN authorization--the Security Council had acted--but simply that by then it was clear that lesser measures of military assistance had failed. The authorization would probably have been made at the exact same time in a Soviet-veto ATL, whether or not the General Assembly had already met.)
Basically, when presidents are convinced there is an emergency and that they cannot wait for some deliberative body to consider the matter, they act first and get retrospective justification--from Congress or the UN or whoever--afterwards. (Compare with the way Congress in the summer of 1861 retrospectively ratified Lincoln's unilateral suspension of habeas corpus.)
[1] I do not of course claim this as an original insight. See, e.g., John W. Spanier in *The Truman-MacArthur Controversy and the Korean War* and David Rees, *Korea: The Limited War.*
http://www.google.com/search?tbm=bks&q="if+the+soviet+union"++"invoked+article+51"