"Perhaps the cruelest thing ever said of Hubert Humphrey was that he had the soul of a vice president." - Susan Estrich
The VP choice has higher stakes for Trump than for Hillary. Ross Perot, the last outsider with a significant campaign, was seriously damaged by his choosing Stockdale. When running as an outsider without a political track record, your first political selection matters more than someone who has been in the system, particularly when you’re someone like Trump who is trying to convince lots of people he is serious.
That said, the presumption of electoral college gains from VP selections isn’t supported by much evidence; even in relatively close elections.
Ryan didn’t deliver Wisconsin for Romney.
Edwards didn’t deliver NC for Kerry.
Bentsen didn’t deliver TX for Dukakis.
Agnew didn’t deliver MD for Nixon.
Warren didn’t deliver CA for Dewey.
Kemp didn’t deliver NY for Dole.
The list goes on.
Shriver didn’t deliver MD for McGovern (which traditionally goes Democrat), and Ferraro couldn’t even deliver NY for a Democrat, which demonstrates the broader point: In terms of a VP choice having the power to buck the predominant voting trend, VPs aren’t that relevant.