When I was working on a CNC degree, I was allowed to use whatever method I wanted to generate a simple plaque. My work flow consisted of a few freeware programs. I started with a bmp image, I believe, and fed that to potrace, which converted my bmp to a postscript, ps, file. I followed that up with pstoedit, which would convert the ps file to dxf files. Oops too much info. Anyway potrace should manage your conversion for you, if you are trying to do it after the fact.
Now… what do you intend to do with the vectorized images? I have a feeling it may not work out the way you expect… (things rarely work the way you expect them to)
I’m not trying to discourage you but it probably won’t work…
Try reading about what other people have done. There are many algorithms out there, each has its advantages and disadvantages. Maybe someone has already solved your problem or maybe you can mix and match other people’s solutions, but you still have to do a lot more research.