You use Adobe Illustrator and start tracing the lines on the normal image with the pen tool Search google, there are MANY tutorials on how to learn it!
Illustrator: http://www.creativepro.com/story/feature/18132.html?cprose=3-46 Photoshop: http://www.heathrowe.com/tuts/vector.asp
If you need a PERFECT vector file then you need to get it traced using Adobe Illustrator or Macromedia Freehand. There are certain softwares and plugins also available that can convert a raster into vector but what I have seen with experience that they donot deliver PERFECT result. They are very quick but results will vary depending upon the complexity. As far as we are concerned, for our projects we get them traced in Adobe Illustrator by experienced artists.
To convert images (.tiff, .jpg, .gif, etc.) into vectors, you can either trace the graphic in any program that works with vectors, either Freehand, Adobe Illustrator, Corel, etc. or use a tool found in "Tools" in Freehand. The tool looks like a Magic Wand, it is called "Trace", you can choose that tool and "click" and "drag" like if you were highlighting... a vector image will be made on top of your .jpg! I suggest you "Group" before you move the vectors, for them to move equally. The outlines will not be perfect, however, it might work for you.
What about inkscape?.. can't it do the trick. I have not tryed it, but believe i read about it a couple of years ago
I belive that inkscape can do the trick.. i am not sure, and have not tryed it myself. but you can read more about it here... www.inkscape.org/wiki/index.php/Tools" (sorry i am not allowed to make "llive links" yet)