Sparda Must've split the last of his powers evenly to his sons after the fatal battle with mundus, and gave them both demonic weaponry in order for them to be able to stabilize their demonic powers, while their mother's necklace stabilizes their angelic powers. Dante may come off as a rebellious, smart mouth, ass-hole, but he is compassionate about a lot of things, he undoubtedly has a good side, which is why he has both white and black patches of hair. Vergil on the other hand, is still a mystery. The fact he has full white hair could mean he decided to focus more on becoming a full demon, or angel.
Just a Theory