Some tips, when defining the nose, unless its a profile shot or an extreme angle, stay away from lines on the side. Focus more on the point of the nose and nostril area. dont even try with the bridge of the nose straight on, that should be taken care of with a highlight, not a line.
for teeth it is much easier to "draw in the negative space" meaning the dark part of the mouth instead of trying to define individual teeth. So you're really vectoring the dark shaded part and just leaving the teeth white. gives a much more appealing look.
Here's a simple vector I did a little while back to show what i mean about the nose and teeth:
http://i828.photobucket.com/albums/z.../vectorsig.png
I realize the perspective is different in mine more the nose (mine is looking up at here, and yours is more straight on, maybe slightly down) but either way it'll still work out better for you.
