About Vergil..
When he got the Force Edge he didn't seem all that powerful to me. I mean yeah, he is the last boss and he is hard as hell to beat at first, but that's only cause he's the final boss.
Storyline wise, Dante didn't even break a sweat to beat him. "Come on, get up, is that all you've got?"
And that's because he didn't get the amulets like Arkham did. Though unlike Arkham, he would probably get all of Sparda's power like Dante did in Devil May Cry 1.
But in DMC3, the only one who managed to get a bit of Sparda's power (and lost it afterwards) was Arkham. Vergil just had one cool Devil Arm to add in his arsenal.
Arkham had all the requirements, but not Sparda's blood. Therefore he got some power out of Force Edge but that was it.
Sanctus did not get anything. He just had the Sparda sword. Even his moves where the same with the 1st battle. He could just block with the sword, and do a couple of moves with it (and one can even be bustered
)
And by the way I'm curious as to whether can anyone take Sparda's power now, as Dante already did in Devil May Cry 1. It would be kinda lame if he was weaker without the Sparda sword, but at least in Devil May Cry 4 he wasn't weak at the least. Also, no mention of the complete amulet. Does Dante wear it in Devil May Cry 4? Has he hidden it somewhere? Does Trish have it?
As for Dante's personality. He has become soo powerful and has lost the only people he had great feelings for. (His parents, his brother, even by his hand, and yeah he may be living still, but Dante thinks he killed him, Trish, for a moment or so). Therefore he's become, ehm "colder"? I don't know if this is the right word. But you see him in Credo's death. Credo is dying behind him and Dante is like "Check it out, it's got wings". Not that he knew Credo to feel anything though. But you get my point. In the end he did want to accept his dying request anyway