So I went to the Devil's Lair (DMC forum) and reread their summary of the DMC 4 novels and I'm convinced that Nero is Vergil's son. Here's why, the novel tells DMC4's story (as depicted in the game) with added scenes. Deleted scenes if you were. Since nothing in the novel contradicts the game its pretty much solid.
Vergil had a good time with a prostitute and she got pregnant. Vergil probably didn't know she was pregnant. All you "Vergil wouldn't do that" go blow yourself!
Vergil is power hungry because of his mothers death but he's human and humans get horny DEAL WITH IT! The prostitute left Nero at Kyrie's door so he was raised with them.
Nero's arm changed when he got attacked protecting Kyrie. The video game booklet and Novel confirm this. Why does Nero have a demon arm? Blame God/DNA. Yamato being Vergil's sword probably contains Vergil's ambitions and it reacted to Nero's strong feelings to protect Kyrie. The Blue ghost is Nero's soul physically manifested.
IT MAKES SENSE, this is basic Japanese anime thinking folks. Those who dislike Vergil being Nero's dad are foolish. The novel author Bingo something said DMC4 had deleted scenes for various reasons and the novel brought them back. Bingo was working on the game so him quitting doesn't change what he knows.
Concerning the age discrepancy US studios have been known to age anime characters because Japanese have a unnatural liking of 17 and younger characters. Want proof?! Look at DBZ's Son Gohan, his Japanese age is different from his US age, fact!
Nero is Vergil's son period!