Just a few things.
1. This leads me to believe the arm is Nero's own arm, NOT Vergil's, since Vergil's arm would only be blue, not red AND blue. I don't think Vergil is 'inside Nero's arm', because how can a spirit be housed inside an arm? It doesn't make sense.
This theory came about from the line 'Power, give me more power.' From the day that Nero got the injury on his arm he heard that voice inside his head and his arm began to change. There is definitly a conection to Vergil but I don't think that it has to do with his arm, I think it's related to the Yamato.
4. He may have been intended to be a half-demon like Dante and Vergil, since his concept art shows him sporting a full-body Devil Trigger (see my avatar, that's it).
I think this was meant to show the design of what the full specter behind Nero looks like. In character design you always make everything there is to make even if the audience never sees it or knows of it. I think that it's also meant to show what the rest of the body attatched to that arm would look like.
I read that only full demons and half-demons can have their own full Devil Trigger, so, since Capcom created full-body DT for Nero, that might mean that he's a half-demon, not a quarter demon like he should be if he's Vergil's son.
I don't belive this is true. I still think that he is a decendant of Sparda but not a direct one like the twins.
- Sparda had a child with someone other than Eva. Probably not possible, since he died before Dante and Vergil were born, I believe.
There's been hinting that Dante knew his father.
- Perhaps he wasn't ''left on the steps of the Order'', (I think it sounds suspicious) but he was created by somebody, perhaps by Mundus.
You mean like Trish? Trish's age is a bit ambigues but that would make him older that her since she was created to snear Dante which implies she was created in a recent period to DMC. But if indeed created for what purpose?
Devil May Cry wikia states: ''Devil Trigger (often abbreviated as D.T.) is a special technique possessed by demons and half-demons which allows them to release their full power.''
Of course, I don't consider that line to 'prove' that anything less than a half-demon can't DT.
That would be my argument.
Nero himself has stated that he belives that spectral image behind him is his demonic true form, no implication of been someone else altogether. I belive that his Devil Trigger is himself an noone else, that the precence of Vergil comes from his sword, not the arm.