I would have personally had Vergil and Dante fall out just before fighting Mundus leading to a short fight in which Dante wins... Vergil gets ****ed and as before, thinks Dante had portrayed him and leaves through a portal, telling Dante he is on his own. Dante pushes along alone and fights an immortal Mundus in his human form, getting SMASHED by the Demon King. Just as Dante is about to die, someone cuts off Mundus' source of power... who is it? Low and behold, Vergil. He stabs Mundus in the back and tells Dante that although they disagree on what should be done with humans, he wouldn't let his brother face their mothers murderer alone and explains that he still loves him but if they take Mundus out, they will go their seperate ways. They battle his "Transformer mode" *facepalm and seemingly defeat it but Mundus transforms AGAIN, this time into a hybrid of his human vessel and true demon form ALA slightly like DMC1 form but more complex and drags them into the REAL hell, where he has access to his power. After an enormous battle, Vergil is thrashed around and as he is about to be killed, Dante, in desperation, unlocks his full Nephilim powers and is able to battle Mundus properly, maybe with the help of Vergils power or Vergil throws him his amulet? As he defeats Mundus, he begins to be sucked into the real world and tries to save Vergil, but is unable to as the dying Mundus grabs Vergil and takes him into the void and Dante is returned to the real world where Kat is waiting anxiously. Deeply saddened by the loss of his brother, Dante could actually shed a tear and looks out at the city with Kat, vowing that even if there is the slightest chance to save his brother, he will not leave him to the demons and will protect humans from demons at all costs. Kat could hug Dante to comfort him and tell him he might not be human but represents the best humanity has to offer. Cut forward to Dante fixing his house into Devil May Cry and a last scene of Vergil standing still in the deepest pits of hell with Mundus' voice echoing words in the background. Vergil starts glowing and as the camera pans in on his eyes, they open and are changing to - Nelo Angelo!!! Personally - a lot more of an emotion arc for the brothers!!
EDIT - Vergils downfall then takes place with Vergil fighting his inner demons and breaking from Mundus' control but going down his own dark path instead!!