Well, I've watched th video, repeatedly, and that aura does NOT look like Nero/Nelo Angelo. Plus, Vergil never told people to give him power, he took the power that he felt was rightfully his - Sparda's power.
As to Yamato, it IS originally SPARDA'S sword that he gave to Vergil, so it would make sense as to why the Order needed the Yamato. If the being with Nero is Sparda(which is still my theory, Nero is too old to be a reborn Vergil), it would probably be a Sparda that is instinctive and does not not yet remember who or what he is.