Written by Joseph Bernstein and Katherine Rosman The night before the biggest assignment of her life, Usha Vance stayed up late with her husband, JD Vance. In their rooms at the Pfister Hotel in Milwaukee, they went over each other’s speeches to the Republican National Convention. In keeping with many intellectual and professional endeavors over the course of their relationship, which got its start at Yale Law School more than a decade ago, they approached the task as a couple. Their lives were about to undergo a dramatic change. Two…