It's really not possible, but you can achieve a decent result with a little tweaking of your layers...
Not great subject matter I know, but it was an exercise with cs2 that carried on. Not my pick of colours either!
It involves lots of layers using the 'threshold' option and then cutting back and re-colouring.
As you can see...it's not vector at all...very jaggy...but on a larger image this process may work.
Looks better than my original...doh!
Aqua-Soft Forums