Hi Rania,
You already have started a thread on HDR capture vignetting. It’s better to add to that thread than create a new one if you have questions on the same subject.
Anyway, your question is confusing, because the fisheye_corr.cal file is to adjust distortion, not vignetting. You also started a topic on distortion correction. Again, it is better to add to an existing post, so the same people can respond.
Creating a circular clipping zone in pcomb is fairly straightforward, if that is all you want. Something like:
pcomb -e 'Cx:xres/2;Cy:yres/2;R:min(Cx,Cy)' -e 'cf=if((x-Cx)*(x-Cx)+(y-Cy)*(y-Cy)-R*R,0,1)' -e 'ro=cf*ri(1);go=cf*gi(1);bo=cf*bi(1)' input.hdr > clipped.hdr
should work. If you want the circular area to be smaller or larger, you can adjust the R constant.
-Greg