So, first of all, I would like to say that the Tupac Shakur trick was indeed just, that, an old lighting trick that produces a video by reflecting light from a projector. This was not actually a 3D projection, which is what the Hatsune Miku projects actually are. They use a deferred lighting method that creates a 3D projection of the game software character (famous for her dancing and very recognizable singing voice). You can see the Miku character below, which will show you the difference between the techniques, perhaps the most notable of which is the amount of movement the character is capable, at times moving all the way across the stage, whereas the hologram is limited to the area where the projector can fill with light.
Perhaps more interestingly is that many people, both kids and adults, cannot tell that Miku’s voiced is an entirely synthesized reproduction of voice. Many, while they realize that her voice is synthesized, still believe that the voice is that of a human’s, and not a computer’s. See below for that.