Well, let’s see here. As far as I’m concerned, they’re both art. the difference though, is that one was made using materials owned by the artist, the other was made using materials of the artist on a building that is owned by another person who may not want his building to have that work on it.
It’s not vandalism because it isn’t modern art. It’s vandalism because someone that isn’t the artist owns that building where the artist placed his work without any consent.
Flawed logic is flawed.