Many times when a student is just getting going in digital art, especially vector, a lot of the same issues come up. One of the first stumbling blocks is shape creation. We see a lot of very simple shapes used for highlights and shadows, that would not usually be employed if the subject was executed traditionally by the student. This can look good, but its not necessarily what they would do if they had more practice.
The red onion above was done in very simple shapes, without much thought going into it regarding the translation from reference to execution.
When doing a full illustration, after we figure out the composition, value and elements, its time to start rendering. Like most illustrations, the rending should start with a tight drawing. What's particular about vector art is that shapes must be though of as closed, working by themselves and in unison with their neighbor, whether next to, or on top of. At first, its a good idea to nail down a sketch in line art, putting most of the creativity in the drawing. Then take it into the computer and just replicate. As projects go by, less and less ironing out needs to be done at the sketch stage and more creative shape making can be done on the fly.
The shapes in this onion were made to reflect its texture more closely. Still, a lot of people work very simply and it often looks very, very good. Things don't have to have a ton of information in them to be successful. This is just one way of seeing things. I look at vector art like I see painting because I tend to render graphically whether its traditional or digital. The biggest difference is that here, I'm building each brush stroke, instead of actually painting them.