I'll omit the long background --- suffice it to say I did this once and need to do it again, but can't recall the toolchain / workflow I used.
I _think_ it was Macromedia FreeHand 8, and that I used a plug-in w/in that program, but I can't recall (perhaps someone else could remember what tool I might have used?) --- anyone have a suggestion other than rendering the drawing as a half-toned bitmap and then tracing it? Or re-drawing the image on top of such a bitmap?
There are several methods. You could do a research for the term "vector halftone" on Google. There was an article on bittbox.com some time ago.
They mentioned some web based services and the plugin Scriptographer
Several solutions have also been discussed in this forum - search for posts by JET
And then there's the plugin Phantasm CS that can also convert your raster or vector art into vector halftone dots.
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