If you want to get nitpicky, Hatsune Miku is a voice synthesizer and character design that anyone can use, whereas VTubers usually have one person who "is" the character. A VTuber is basically the video equivalent of someone who uses a fake name on an internet forum; fans generally think of them as real people with coherent personalities. (Whereas any personality Hatsune Miku has is more like a loose convention among all the different artists who use the character.)