I think this point has probably been made elsewhere, but from a rainy day at my beachy retreat it occurs to me to point out that one really ought to look at the selection of a Vice Presidential nominee as something where the substantive merits are important. Of our eleven postwar vice presidents (Nixon, Johnson, Humphrey, Agnew, Ford, Rockefeller, Mondale, Bush, Quayle, Gore, and Cheney), four have gone on to become president and three more have gone on to become a major party presidential nominee. That's by no means a perfect batting record, but generically speaking becoming vice president is the best means of going on to become president. Under the circumstances, it seems foolish to advocate for someone or other purely on the grounds of political expediency.
Indeed, I suspect this is part of the reason that "veepstakes" conversations tend to get so annoying. Much like "electability" controversies during presidential primary season, people are playing without putting all their cards on the table, and tend to coincidentally get the result that the person who they think would be the most politically savvy pick is also a substantively good choice.