I'm a bit torn on this question.
Yes it is vandalism. but if it is done well, it is art. Just painting a stylized squiggle is not art (to me).
but for most taggers, their goal is not about creating art, or making a statement (unlike Banksy). it is about ... tagging. Putting their mark to say 'Look how cool I am. I'm a rebel. Chicks will love me for this.'
They don't understand, and don't care that they are damaging someone else's property.
But for those who paint something that displays talent, or thought, or promotes a laugh or thoughtful reaction, I can soften my condemnation of the crime.
I guess it comes down a lot to where it is, what it says and how well it is done.