Why is Vance even on the list?

Mostly it's because Trump - or more accurately his son - believes that Vance is a very effective communicator and salesperson for their political ideas.

Some campaigns look to the Veep role to provide balance to the ticket. Geographic balance, demographic balance, experience balance, age balance or even ideological balance to unite groups within the party. IOW, the Veep brings some biography to the ticket that the nominee might lack.

But another huge role of the Veep is to be an active campaigner. Like the nominee, they're on the stump and doing events and fundraisers and doing TV appearances and all the stuff that goes into a modern campaign. The campaign (and again, especially Don Jr.) thinks that Vance is very good at doing that.

They might feel that's the most important role for the Veep this campaign cycle. They certainly no longer feel they need a bridge to either the establishment or the evangelical voters (which was Pence's role last time). They probably don't feel like geographic balance is really going to be a factor in this election (and Vance is from the midwest anyway). Demographic balance might be valuable...except that the states are really likely to play out (Michigan, Wisconsin, Pennsylvania) are all whiter than average - which sort of limits the potential upside of a non-white running mate. If North Carolina or Georgia looked like they were more in play, Tim Scott might have a better chance in the Veepstakes....but it doesn't really look like that's where this campaign will be won or lost. Rubio would add a Latino to the ticket....but again, the midwest is likely to be the real battleground, and Trump simply might not trust Rubio to be as good a salesperson for or partner in his agenda.

So Vance is on the shortlist. He ticks the boxes for a midwestern young person that can energetically sell the campaign line, and seems to do well on the campaign and on TV.