This was in the last update from December 8th:
United’s International Network Planning team is still working on a few fleet-assignment details for the summer 2018 season, and our Domestic Network Planning team is putting the final touches on the 2018 summer flying level. Because of this, I do expect one additional vacancy to occur in early January, prior to the opening of annual vacation bidding.
and as a reminder this is what January's vacancy last year was: