Sen. J.D. Vance has a major vulnerability that gives Gov. Tim Walz a big opening at the upcoming vice presidential debate, according to a former speechwriter for George W. Bush. Ex-Bush insider David Frum wrote in a piece published in The Atlantic on Sunday that Vance, Donald Trump’s pick for the Republican party’s vice presidential nominee, is made weak by his “thin skin.” How Tuesday’s debate turns out could “hinge on whether Tim Walz can exploit his rival’s greatest weakness,” Frum wrote. “Kamala Harris used Donald Trump’s psychic weaknesses against him in their televised debate on Septembe…