The video shows a meeting between the Chinese delegation and some senior American officials, but I don't see Kamala Harris, the vice president. Is there a reason why Kamala Harris wasn't present during the bilateral meeting between China and the U.S. during the APEC summit? Did the U.S. government give an official reason for that? I thought that the vice-president was the second most important official in the U.S. government, but it seems that for some reason Blinken is more important, as secretary of state.
I don't think qualifying the VP as the 2nd most powerful official, at least in this context, is accurate. The amount of influence a VP has on policy has been historically highly variable, depending on the VP's ambition and their relationship with POTUS.
Blinken, as the Secretary of State, is the lead here.
By statute, VP's official powers are quite limited
The Constitution names the vice president of the United States as the president of the Senate. In addition to serving as presiding officer, the vice president has the sole power to break a tie vote in the Senate and formally presides over the receiving and counting of electoral ballots cast in presidential elections.
Today vice presidents serve as principal advisors to the president, but from 1789 until the 1950s their primary duty was to preside over the Senate. Since the 1830s, vice presidents have occupied offices near the Senate Chamber. Over the course of the nation’s history, the vice president’s influence evolved as vice presidents and senators experimented with, and at times vigorously debated, the role to be played by this constitutional officer.
See also: Constitutional roles
Now, as to why Harris specifically wasn't there, that's for another answer. But her job did not necessarily expect her to be present and there may not be much else to read into it.
She also did seem to be busy doing other things at APEC, and (state gov) : Vice President Kamala Harris will chair the ABAC Dialogue, so maybe it was just a scheduling issue, esp. as the Xi-Biden meetup was a very late addition.