I know I'm not the best, nut you can take what I have to say for what it's worth. I'm a big fan of vector art, because it's nice and simplified. You don't have to get complicated with the colors and most of the lines are clean and smooth.
One thing about it, however, is that the perspective is a little off; the body is pointing one way, and the corresponding direction in which the head is facing doesn't match up. This goes in conjunction with what Röcki said about the eyes. Most of the art books I've read used simple shapes as reference points to help out with keeping your perspective consistent throughout; personally, this is where geometry comes in handy...
If you know how CAT scans work, they pretty much "slice" your brain up into millions of 2D images, then puts them together to create a 3D rendition; this is pretty much what I'll be showing you in the below pic, using the vertical center of Riolu's head as a "reference slice."
Another thing, that isn't very big, but may be a world of difference can be to make your picture less, well... boxy. It becomes apparent after the many sharp angles you see. Just a suggestion...
The red lines indicate reference lines
The green lines indicate correctional lines (at least, what -I- think would be correct)
Hope this helps!!
