The only way Vergil would get in would be if Dante gets in now as DLC and then for the next Smash they bring him back and then go for Vergil to break the "rule" of a 3rd party series having 2 unique characters.
And that won't happen with DMC, even if I'd love that. If that ever happens it will be either Sonic or Street Fighter, I think. Maybe Mega Man.
As for Vergil content, if Dante is one of the challenger packs, Vergil will be, for sure, a spirit, and a stage cameo if they decide to go that route with Devil May Cry.
Possibly a Mii Costume. Also I could see Dante's Final Smash featuring Vergil, similar to Robin's Final Smash I guess. In that case it would probably replicate this scene from DMC3 (spoilers):
If Dante gets in I'd love to have Bury the Light as one of the included songs. I know the chances of this are almost none because of how recent is the song and all but hey, who knows.