RE: How should a VP be replaced?
When any setting VP runs for President there should be one or more nominations for his replacement made at the same time his nominations are submitted. The nomination of the VP for President would include nominations for his replacement should he win the seat. These nominations should then appear on the ballot coincident with the setting VP's and voted on as a contingency if the VP wins the election. The usual write-in provisions should be included.
This would 1) assure that there is no gap in representation, 2) add no cost to the process.