There are some programs to help approximate a raster into a vector format but they all are not perfect. You have to keep in mind, you are trying to convert a bunch of black pixels (which define a line) into data that some CAD program can use (i.e. Line, on layer 0, color black, continuous, from point 3,4 to 17,23) The programs "try" to do it by themselves and then afterward, you need to interact with it to "correct" its wrong calculations. I have used Autodesk's version (I think they now call it RasterDesign) and it works okay.