I have never voted based upon who would be better to step in as VP, but I did not vote for McCain specifically because I felt Sarah Palin was incapable.
I find both Walz and Vance capable, so this will not be part of my criteria.
However - I don't view Trump as capable in the first place, so if he wins, I have no problem with him croaking and Vance taking over.
I still want to believe Vance is better than the person he portrays these days.