I don't like the idea of bringing Vergil back after he had a very tight timeline. You can show the DMC3 manga storyline, you can show Vergil's trip to Fortuna, or Vergil's time under Mundus leading up to his appearance on Mallet Island. That's it. Accept it, Vergil kicked the bucket and was harvested by the Order of the Sword. I SUPPOSE there could be a story hook involving Vergil's soul in Hell, or inhabiting the Perfect Amulet, but besides that.... Rest in peace. You died.