I suppose I should list my conclusions on Nero's origins as my final post.
Nero is not:
1. Nero is NOT Vergil's son, since Vergil would've conceived Nero at the age of twelve. Besides, he doesn't care much for humans, and he certainly wouldn't want a - for the most part human - child.
2. Nero is not a third son of Sparda, since Sparda died before Eva did, and Eva died when Dante was eight. So even if Sparda had died at the same time as Eva (29 - 8 = 21), Nero would have to be 21, not 17.
3. Nero is not Vergil's or Sparda's reincarnation, since reincarnation means that ONLY THE SPIRIT goes into a different body, making it impossible for Nero to have the blood of Sparda (and white hair).
4. Nero is not Dante's son either, for the same first reason noted at option 1. Besides, Dante isn't the kind of person who would have a child, IMO.
Nero could be:
1. He could be a weaker clone of Sparda, left at the orphanage in Fortuna by the person who created him, perhaps hoping that the Order of the Sword would take Nero in. And no, clones don't have to look identical to the person who was cloned.
2. He could be an experiment, somehow created by use of Sparda's blood. It could be that, unlike donated human blood that comes into the bloodstream of the recipient human, the blood of demons doesn't disappear after a while.
3. He could be the son of one of Sparda's relatives. Why? Because Sanctus said ''a descendant of Sparda's blood'', 'blood' meaning 'kindred/relatives' (look up 'blood' at merriam-webster).
This means that Nero could be a half-demon like Dante and Vergil, which is possible, since there is concept art of Nero having a full Devil Trigger body (my avatar).
4. It could also be that Nero (especially his arm) was a character created by Capcom without much thought put into it.
We know that Nero has the blood of Sparda. Quote: ''I must salute a man who carries the blood of Sparda'' - Sanctus. Also: ''a descendant of Sparda's blood'' (also a quote from Sanctus).
Since Nero has always had white hair, it makes the premise of him being a descendant of Sparda (OR a clone/an experiment created by use of Sparda's blood) quite likely.
As for Vergil's connection to Nero:
Vergil's soul is inside the Yamato. I think Nero heard Vergil call out ''Power. Give me more power!'' from inside the Yamato. Nero was in Mitis Forest at the time, which isn't far from Agnus's lab, where the Yamato was. Vergil echoed that line, probably because the Yamato was broken, and Vergil's power was weak because of it.
Though it's very odd that he only heard Vergil when his arm changed into the DB. It makes me think Vergil is inside his arm, but that makes no sense to me... how can a spirit be inside someone's arm?
Since there's no answer to the question who Nero is, and Capcom haven't given any answer either, I think Capcom screwed up. They didn't give us an answer, BECAUSE THEY DON'T KNOW THE ANSWER EITHER.
They're creating the reboot DmC because of that.