At the time the Soviet ambassador to the UN, Yakov Malik, was boycotting all UN meetings. So he wasn't present to use the Soviet veto.

The reason he was boycotting was over the Chinese delegation. At the UN's founding the Republic of China received a permanent seat on the Security Council. But by 1950, the ROC only included Taiwan and a few small islands. None the less they still held the Security Council seat. Needless to say Stalin wasn't particularly happy about that. He wanted the Chinese seat to go to Mao's People's Republic of China. On January 13, 1950 Malik proposed booting the ROC delegation and accepting the PRC. The vote went 6 to 3 for keeping the ROC.

So following Stalin's previous instructions, Malik stormed out and began a boycott of all Security Council meetings until the PRC was recognized as the legitimate Chinese government.

When the votes on Korea took place in June 1950, the PRC didn't have any vote, let alone a veto, and the Soviets didn't have anyone in the room to exercise their veto.

The PRC wouldn't gain official UN representation until 1971 when the PRC finally had amassed enough support to become the officially recognized China