Soldato
It's still vandalism. If the owner didn't ask for it or want it, and it damages (or alters) the original structure either materially or, in this case, visually, it constitutes vandalism regardless of whether it is any good or not or is worth money or whether people think it enhances the structure or whatever.
It's not.
If you, guerrilla-style, replace and upgrade the glazing on a public library, then would that be vandalism? It, in fact, would not. That's not to say something "wrong" wasn't done, but that "wrong" wouldn't be vandalism.