Quote - I made a test render, and, well, the TIFF 16 does nothing visible with the Vue AA in the z-channel.

You are correct, the 16bit doesn't change anything with respect to the AA of the z-channel. It simply saves the z-depth map with the full 16bit gray scale info that Vue uses internally. Unfortunately, this does't help for the trans map artifacts you mentioned (those have bugged me too on  several occasions, with transmapped hair for example). However, it does help significantly in my experience when you have long continuous transitions that are getting de-/increasingly blurred. For example a ground plane or a terrain or walls. The blur on those will be far smoother when using 16bit z-depth. That's what I was referring to.

Ciao
TD