Anyway, Vergil will most likely watch Dante at first, then send Kat to recruit him at the beginning of the game. Dante will say no to Kat at first, then agree to do a test mission, to see if he can get along with The Order. This will most likely be the factory, right before the club mission where he tries to rescue Kat. He rescues her, then somehow Vergil gets involved because he was angry that Kat got kidnapped. He then defeats Mundus (with Dante's help), takes his power, and then turns on Dante because he became corrupted. This is just a big (and most likely wrong) theory, but I'm just going to go ahead and say that this is how it will more or less happen.