vex or lego cnc

My curiosity has overcome my fear of probable embarrassment so here we go. I have several VEX robotics kits (erector set with a PIC brain) and now a couple Lego NxT kits. We've done robotics competitions with both kinds of kits, as well as the big FIRST robotics 120 lb unit. And have done programming to get the robots to do autonomous stuff, so we have a little "C" and "NxTG" programming capability. But now I want to use these platforms as teaching platforms for CNC. I've googled lego cnc and vex cnc and there are those doing some work, but I'm having trouble figuring out the software steps to get the VEX or NxT micro controllers to do actual CNC. The little NxT motors have encoders built into them, so we could actually know how far we were moving, I'm just stuck with what to do beyond writing code for each move. As an example, I'd like to hook these little motors to an etchasketch. I can see how one might draw a square, but I'd like to draw a circle. Do we just have to learn more "C", or is there a way we can get closer to "real cnc" - gcode and an interpreter for the NxT or VEX? Thanks.