[Vergil continues to eat while he listens to Nero's recounting of how his arm came to be, and hums thoughtfully at its conclusion. It sounds as though perhaps his healing factor was attempting to manifest, but it was not able to do so completely. The reasons for why it would not awaken completely, if Vergil had to hazard a guess, likely stemmed from Nero's ignorance of his heritage. Not knowing of the demonic blood that flowed within his veins, he wouldn't have any idea of the power within him to properly awaken it. It is little wonder that it's taken him this long to begin truly tapping into the full potential of his abilities. Had he never crossed paths with Dante and everything else that unfolded, it's likely it would have taken longer. Assuming that it ever happened at all, of course.]

Given the circumstances, you did the best you likely could have. [Which is not some form of awkward praise from Vergil, so much as it's his observation of what Nero's shared with him about what happened when his demonic powers first manifested. Vergil finally reaches for his own hot chocolate and takes a sip. Unlike Nero's there's no spicy kick to Vergil's; it's a white chocolate with lavender instead.] I assume when you came to possess it, Yamato was able to awaken more of your power.

[It is, after all, what it did for Vergil. It would only make sense that the blade would do the same for Nero, especially if it was willing to submit to him as a temporary master in Vergil's absence.]