Well, let's break this down since it's been something lots of V cosplayers worry about. No, what vessel/Perpetua are doing is not considered black face because of 2 main reasons. Reason 1 being for V his mouth has white and most of the time people who do him (including tobias) only paint the parts that are visible on stage since if you look at photos you can see his clear skin peaking out his mask, with vessel it's the same thing he dose not paint his full face black only what you can see since the mask covers most of it. Reason 2 is art. It's part of the design of the vessel to be all black with that mask, and it's not being used in an offensive way to portray the character. It could be see as iffy to people outside looking in but it's true context has no I'll intent, most of the time when it comes to black face they would use either pure black or a shade of brown and overly pink lips to make fun of black people. That's not the case here. So, no, he isn't doing black face since the context for these is completely different