[The expression doesn't come as much of a surprise, nor does the...terseness of the reply. It's why Vergil doesn't believe he's necessarily said anything particularly wrong or crossing a line, nor does he take it as a personal offense. Vergil also knows he wouldn't react much better in his youth if the topic of conversation ever came up. If anything, Vergil's reaction would be worse and even harder on himself than Nero appears to be towards himself.]

That is the most direct way, yes. But the more you develop your repertoire, you will have more ways to push yourself than just maintaining that form.

[Learning to manifest more than one blade at a time will help his endurance just as much as maintaining his devil trigger will, and will likely be of more interest to him anyway given that invites a bit more thought.]