This is a paradox of narcissism: The narcissist wants only to elevate himself, yet he needs to work with, cajole, and convince others of his greatness. Trump has long proven willing to make any deal to advance this goal—that is, to balance his narcissism with his desire to win. When he needed to please Evangelicals to have a shot at the presidency, he chose Mike Pence. When he needed to get more ideological conservatives onboard, he worked with the Heritage Foundation and approved of a pre-selected list of Supreme Court nominees.
Yet, when it came to choosing his running mate this year, Trump abided by none of his previous guideposts. Had he picked Nikki Haley or Marco Rubio, he could have attracted more female voters or Latino voters, respectively, and both options would have appealed to voters looking for a younger, establishment Republican to moderate his anti-democratic tendencies and, if necessary, complete his term in office.
Vance does none of those things. He comes from a state, Ohio, that Trump was going to win anyhow. He brings Trump no benefit in terms of attracting voters who might somehow still be on the fence or looking for an excuse to vote for Trump despite all his inadequacies. So why pick him? Trump was probably seeking two things: Someone who will never challenge him, like Pence did, and someone who will never soak up any of the limelight, making Trump look like a sucker.