You all are forgetting an important item that was missing from the rebellion in DMC4 and that was the Golden Ribbon Vergil used to tie the sword to his waist. Now this my be a small thing but it does leave a little clue or maybe.
Btw that post I made was only the most likely thing to happen if he returns. I'm not stating that as a fact... Well except the part where he did tell Dante that no one can have his amulet, no one probably meaning Dante as well. So its obvious they will fight again for due reasons.
As for Nero, Vergil is a very selfish person and he admires his father so I highly doubt he will give away his prized sword just like that, mainly because it was used by his father and we all know Vergil wants to be as much like Sparda as he can.