HOA board member wants to resign. Can she vote on the appointment of an interm replacement before she reigns
That cannot be answered without reviewing the CC&R's and bylaws.
Reigns? I guess you mean "resigns." I would say the vacancy doesn't exist until the resignation is not only irrevocable, but has actually taken place. So, I'd guess the answer is no, but as Mr. McCormick says, a careful reading of the HOA's governing documents might give a different interpretation.