you can check out Qgis [Sextante Toolbox][1] in plugins repository. it has lots of geoprocessing alagorithm in it.
SEXTANTE is a spatial data analysis library and a powerful geoprocessing framework.The main aim of SEXTANTE is to provide a platform for the easy implementation, deployment and usage of rich geoprocessing functionality. It currently contains more than three hundred algorithms for both raster and vector data processing, as well as tabular data analysis tools.
and there is a good answer for classified vectorizing [here][2] from @nhopton. follow his direction to achieve what you want.
i hope it helps you... [1]: http://www.sextantegis.com/index.html [2]: http://gis.stackexchange.com/a/27145/3071