every body just be queit and listion I'll clear everything up for you, one thing that could have happened was when yamato stabed nero, it still has some of vergil's blood on it when he was in the control room in the ten-ma-garu. but back on topic, nobody nows how he got the devil bringer, the devil bringer could be a medium like the hell gates, combined with yamato, it was just enough so vergil could come through in an astral form, and attack, nero is a desendent of sparda how I don't know, nero knew that if both credo and agnus became demons then the leader of the order must be too. he knew that he would most likely need yamato to stop him. if he was all human at the begining of the game, then most likely the devil bringer slowely began making him a half demon.