This is a little theory of mine. Let's assume that after Dante defeats Nelo, Nelo is called back and punished for his failure. However when Mundus is defeated Nelo is able to escape into the human world.
However the plan had a little backfire. When Vergil finally escaped he found himself with amnesia, in coma and with one demon hand.
While he is unconscious the Order use chemicals, devices or whatever they had there(they do have a mad genius) to give him false memories and try to win a demon to fight for them.
When he regain consciousness he thinks he is Nero, had a beautiful life with the Order and had a demon hand due to who-knows-what in his system recently.
The voice echo "Power, give me more power" is his true self trying to surface and as the game progresses he will became more Vergil then Nero and finally he would regain his identity.
However the plan had a little backfire. When Vergil finally escaped he found himself with amnesia, in coma and with one demon hand.
While he is unconscious the Order use chemicals, devices or whatever they had there(they do have a mad genius) to give him false memories and try to win a demon to fight for them.
When he regain consciousness he thinks he is Nero, had a beautiful life with the Order and had a demon hand due to who-knows-what in his system recently.
The voice echo "Power, give me more power" is his true self trying to surface and as the game progresses he will became more Vergil then Nero and finally he would regain his identity.