Jeroen
There are several way's of doing this i'm going to tell you two: 1. Use Adobe© Photoshop/Illustrator pen tool and trace your lines; 2. Use Adobe© Illustrator and use the live trace tool on your images this will automatically vectorize images.

DavidEF
The best way I have found is to use the free SVG editor called Inkscape. You can trace any bitmap into a vector, in SVG format. It also gives you the option to separate the colors into individual layers, which you can edit independently.