Not to beat a dead horse, but specifically Rule 9 of the Rules of the Republican Party provides that:
(a) The Republican National Committee is hereby authorized and empowered to fill any and all vacancies which may occur by reason of death, declination, or otherwise of the Republican candidate for President of the United States or the Republican candidate for Vice President of the United States, as nominated by the national convention, or the Republican National Committee may reconvene the national convention for the purpose of filling any such vacancies.
It is an interesting question what would happen if Vance fought being replaced.