Nitro - the jaggedness that you seem to refer to is just because of the forum's display that shrinks the image after you've uploaded it. As for realism, from what I've seen the amount of skill needed to pull that off with a vector/illustration program is insane.
Mere mortals (which we usually have to admit that we are), are better served using some kind of poser model, or creating something in a 3D modelling app, and then integrating that into a design done in Photoshop. In terms of the amount of time spent and also flexibility, you'll be better served.
If it's about developing your skill, you will need (as ronmatt said) stay away from uniform colour blocks - use sectioned gradients. Another approach is to actually trace a photo, create your basic shapes, and then try to match the colours from the underlying image. Here you use the mesh gradient tool - an excellent tutorial can be found here:
http://www.lifeinvector.com/index2.html
Go to "Downloads" and then at the bottom of the page you will see
Gradient Mesh Red Bell Pepper Tutorial