Do want Vergil, but ONLY if they can think of a good, plausible reason for bringing him back.
Personally I'd like to see less of the whiny emo douchebag Nero, because he's an appalling character with no real long term development.
However, a lot of fans are gonna want more about Nero, so why not do what they did with DMC2? Split it into two discs, because it's pretty much split anyway, a lot of the fanbase want Vergil, a lot want Nero, bring Vergil back with a DECENT, PLAUSIBLE explanation and then have the story play out on two discs with the player being free to play as who they wish at their leisure.
That'd be the ideal scenario, but then again, asking the guys at Capcom to create a plausible reason for Vergil's return would require them to be able to write something worth a damn. If only there was a really good writing team somewhere in the world who originally worked on Devil May Cry...Oh wait. They got fired for no good reason.