People .... the back and forth sort of speaks for itself.
Evey: Who are you?
V. : Who? Who is but the form following the function of what and what I am is a man in a mask.
Evey: Well I can see that.
V. : Of course you can, I’m not questioning your powers of observation, I’m merely remarking upon the paradox of asking a masked man who he is.
(Here, V says why he made the remark... basically, when saying “who “ he’s saying “really are you serious?” So he then says the “who” is but the form ( person) function of “what” he is . He is a masked man first and a person ( who ) second . Later he further clarified what he means when she says “I can see that”(or Duhh).
So he then says that he’s not questioning he powers of observation ( what she sees ... the person (who) behind the mask( what he is)., but questioning the irony (paradox) of asking who he is ... that’s why he has the mask on! Like asking a blind man “ did you see what just happened a sec ago?”
Evey: Oh, right.(oh yeahhh)
V. : But on this most auspicious ( important, special )of nights, permit me then, in lieu of the more commonplace(because it’s not a more formal place) soubriquet(very important!!...he’s going to give her his aliases or nicknames), to suggest the character of this dramatis persona(dramatic person/ over the top character). Voila! In view humble vaudevillian veteran( he’s a Veteran of the stage), cast vicariously as both victim and villain by the vicissitudes of fate. This visage, no mere veneer of vanity, is a vestige of the “vox populi” now vacant, vanished. However, this valorous visitation of a bygone vexation (Guy Fox is a dead person . He brought back)stands vivified, and has vowed to vanquish these venal and virulent .... etc...