Demons can increase their powers to their full extent If they use It to aid, to help people - for good.
Sparda is called The Legendary Dark Knight for that, and for many other reasons I'm not going through right now.
Dante and Vergil had It easy to increase their power. Half of their existence was human. Therefore they used that in their advantage. Vergil fully accepted his demonic heritage, while Dante accepted his human heritage.
Don't forget Dante didn't knew he was a demon or a human until the Devil Trigger on Devil May Cry 3.
Vergil's human side wasn't dead. He cared all the way for Dante, even though in his own way.