Thinking. If resignation before electors meet I am thinking that the elector votes for Pence could be cast for VP replacement?
Uncertain.
If after then 25th Amendment definitely applies...VP Ford.
If the VP resigns or there is a vacancy, the President nominates a VP, and he has to be approved by majority vote of both houses of Congress. It was a solution passed at a time when our politics was less divided. If the President nominated someone qualified, he would pass easily. These days, both houses would probably reject anyone Trump would nominate.