You could try replacing the pen cartridge with a vinyl cutting drag knife which is just an offset blade in a bearing. A lot will depend on wheter your plotter is strong enough.
I use my router for vinyl cutting and made a small assembly for this, I put some pictures of this in the photo section of
http://groups.yahoo.com/group/CNC_Toolkit/
and have attatched one picture to this post.
Your software needs to account for the offset of the blade as it's always lagging behind the programmed position, typically by 0.3 - 0.5mm
Regards,
Rab
http://www.rainnea.com/cnc.htm