I've watched the final trailer of DMC4 several times. I noticed right before Nero turns into his DT form he is kneeling down and laughing. I honestly believe 100% when Nero turns DT it is Vergil being released from his human flesh prison (Nero's body). Most people probably thought he was crying but if you listen closely it sounds like he is laughing or rather Vergil laughing because he is finally released it just took Nero going into DT to bring him out. I'm not saying I'm right but I think Capcom has something really badass in stored for us concerning Vergil.