“Website down for lack of payment” is quite taunting. Taking it down is fine (I would have), but posting that message there probably not necessary.
Did you not get paid because they made it clear they did not want/intend to pay? You’re sure it was not a misunderstanding? Personally, I would have just taken down the site, not air any dirty laundry, leave it at that and try to work it out.
I’m not sure if you’re really after a lawyer’s opinion (I’m not one), but if you want to get into the official legalities, then you probably wouldn’t win; the verbal contract is difficult to prove, the “vandalism” however, is easy to show. If the client stayed within the boundaries of the law (or, well, if you can’t fully prove they didn’t!), your way of handling things probably would look a little unprofessional.
