Try getting a true emotion to come across with a Vocaloid and then you'll understand my statement. Please keep in mind that this is just one view and I'm not shutting down yours. I'm sorry if tremellos and vibrato sound like total crap on a Vocaloid in my mind. :/ It's one person's view, but I'm also offering this from a traditional compositional sense. Also, sorry if I'm using the term "robotic" to mean "devoid of feeling or emotion," in the generic sense of the term in this context. You can get pitches to match and have deep lyrics, but the manner in which they're executed affect the emotional involvement of the listener.
Besides, Vocaloids are definitely a last-ditch effort for vocals, as I can see it. The robotic vocals can only work on very specific stuff, but it can definitely lack the emotion a human emotion carries with it. Compare Miku versions of songs with their actual counterparts for examples.