i know it's old but i think this will confirm that Nero is Vergil's Son
http://www.capcom-unity.com/carbo/b..._devil_may_cry_news_no_new_game_unfortunately
3. Devil May Cry 4-wise, one of the localization employees I was talking to confirmed to me that Nero IS Vergil's son, and that this aspect was going to be implemented in to the game's plot. However, the decision was made not to, which could set up a good story for the future. I argued that it was cheesy, so we'll see what happens.
http://www.capcom-unity.com/carbo/b..._devil_may_cry_news_no_new_game_unfortunately