Shane Smith, the chief executive and co-founder of Vice, announced on Tuesday that he is resigning as the leader of the millennial-focused company that he built into a new-media giant. Nancy Dubuc, the CEO of A+E Networks, will take the reins, with Smith moving to an executive chairman role at the Brooklyn-based company. Vice has grown into a global brand that includes popular digital and TV properties, including a nightly news show on HBO.