There are many kind of techniques to vectorize a sketch. For complex illustration, you can use tracing technique, or live tracing which are quite complicated in process. You'd probably have to spend longer time in inking your illustration before you scan it and continue the digital process. If your sketched design is simple, symmetrical and appears with enormous geometrical shapes, you can just rely on vector shapes tool (Rectangle Tool, Ellipse Tool, etc.) to get it done.
In this this quick tip tutorial, I'll show you how to vectorize shapes based on a sketch with the help of layer opacity in Illustrator. I think it would work for any complex illustration, but today's tutorial is a bit different. This is because vectorizing always take intensive uses of Pen Tool so it's going to be better if you see a short visual demonstration in a video for that. To get better view, I suggest you to see the videos tutorial in full screen mode.