“What, uh, do we believe, sir?” a young Richard Cheney, Donald Rumsfeld’s aide during the Nixon years, asks his boss. Rumsfeld, played by Steve Carell, laughs hysterically, blindsided by the naivety of the question. Matters of personal principles and ideology simply do not factor into Washington power games. It’s a central concern in Vice, the latest in Anchorman director Adam McKay’s swerve from knockabout man-boy comedy to polemical film-making, but it’s also a question the film desperately needed to ask itself. What does McKay believe? What moral vision is he trying to put on screen? What on earth is Vice…