Necklace used to stop vag from teleporting when he was near you, yes. Now, it does nothing(both in playing experience and test server testing).
No, it never revealed them.
Yes, those always were and still are great ways of dealing with vags
True you can't see a vaga when they are invisible, but that's the naked eye. Check the radar and you'll see a dot when looking in their direction (usually). Though, it never tells you how high one is... or does it?