My understanding is that a vacancy in a candidacy for President or VP after the convention depends on the party rules. Most likely the GOP National Committee would make the decision, but if there was enough time the National Convention delegates could be recalled by the national committee. This would also, IMHO, be what would happen with a vacancy occurring between the election and the vote of the electors. After the electors vote, if the winning candidate dies or becomes unwilling, the election would go to the House of Representatives, with each state's delegation getting one vote, and they might be guided by the winner's party's desires.
If it were left to the RNC to choose a replacement candidate, they would no doubt choose Specter if Chafee were to remain unavailable due to his refusal to re-register as a Republican. /s