Well, first off. It's more of an anime vector than manga, really. Second of all, it's highly cliche.
You have the idea down, decent mapping. The ability to use the pen tool effectively. Etc, etc.
But the big thing here is you can/could be more original. I understand the cartoon style is popular, but it's just so generic to me. I feel like i have seen this exact head a million times.
The next thing i would suggest is studying how anime/manga is actually drawn and try drawing some characters outside of the realm of head textures, to get a better idea of how it actually works. You don't need to do full bodies, but practicing eyes, faces, noses, hair, etc will help big time.
As far as the shading goes, it lacks much depth. Don't be afraid to use grads, air brushes, overlays, etc to achieve great tone and depth.
However, if you are going for a more manga style here, i would suggest getting a tablet or practicing drawing by hand with the mouse a lot. Manga is generally done by hand and not vectored.
Overall, you're on the right track, you just need to experiment outside the cliche cartoon head style.