Since his selection as Donald Trump’s running mate, Senator JD Vance has performed poorly on most metrics by which we judge vice-presidential nominees. He has never been noted for his charisma, and his attacks and insults have rendered him exceptionally unpopular. But he was not picked for traditional political reasons, like electoral strength in a swing state or being the ambassador to a needed voting bloc. It may be hard to recall after weeks of talk about “cat ladies,” but Mr. Vance, as much as virtually anyone else in Mr. Trump’s orbit, has tried to put ideological and policy meat...