If you're not experienced in AutoCAD I would personally recommend Illustrator to do a vector rendering. This is mainly because hatching solid shades in autoCAD can frequently cause the program to crash with increasing complexity of a shade region, illustrator tends to handle those better. There's an easy work around to avoid the crashing but it involves making shading areas smaller to reduce the complexity but it's more extra drawing that doesn't really wind up as part of the final render.
As for whether CAD is actually useful. It's not really designed for this kind of work but it works well enough and I've been comfortable using it. The idea in autoCAD is to attach the image as a reference and then redraw all of the line work using a combination of splines, circles, arcs, or whatever is handy to recreate the render.
These videos I did a few months ago show parts of the process I go through to do these sorts of renders:http://www.youtube.com/watch?v=GE7M7R8dzPQ
^^Music's a bit loud on this onehttp://www.youtube.com/watch?v=09kihFmQL90
^^Shows how some of the basic colors are laid out in layers and applied as hatcheshttp://www.youtube.com/watch?v=L6WPXio73X4
^^when I have linework using a variable lineweight.
Basically the purpose of CAD is for the vector work
. You can export the dwg as a PDF and then put it photoshop at any resolution and size that you want.
Something like what you posted as an example would take an obscenely long time to do it CAD, probably three times as long, but mainly because of the level of detail involved. If you're just rendering a single anime character as some of your other examples are, then the time to retrace them usually ranges from 6 hours to a couple of days worth of work.
For the two picesyou have at the end, the Highschool of the dead render would be the easiest taking probably 3 or 4 hours to retrace the line work and then get the hatching of base colors. Add another hour or so to put it in photoshop and render the gradients.
2nd one is also quite doable but it has a lot more detail. It'd take about a day or two to get the linework redrawn and the base colors hatched in. Add about an hour to two hours of photoshop work for non-solid shading (gradients).
EDIT: If you'd like I can provide some of the source dwg files. Just get AutoDesk Trueview if you need a dwg viewer
I decided to go with the one with Shiro holding the girl's head. Here's a png export I did from acrobat (the originals' much larger)
I haven't finished the drawing in CAD yet... This is about where I am after a day or so
EDIT 3: CAD work laying out all the basic colors is completed.