About the part that Dante said to Trish "he's the one who took the life of my mother, my brother, absurd"
First of all in DMC we didn't know anything about Dante having a brother until that point. From what we could think is that Vergil was taken from Sparda's house as a hostage or escaped the attack of the demons and found a way in, where Mundus bewitched him.
Then DMC3 came and we saw that the 2 brothers have already met before Vergil head off to the demon world. And even before that, they had met in the manga, one year before the events of DMC3, where Dante first saw his brother alive. So the part that demons took him hostage when he was young is invalid.
At the end of DMC3 Vergil throws himself off the waterfall and sees a glimpse of Mundus. Weakened as he was from the fight with Dante he probably never stood a chance and lost to him. Mundus probably used someways to possess him, maybe by giving him the power of N.Angelo in which he could stay in Demonic form forever, and Vergil started losing himself to it..