Here is a couple ideas.
1. Get into a text mode, either by launching the tui desktop or opening a terminal. Become a super user [su then password]
and run vxconf. Then choose the vxconf option..When choosing resolution in vxconf choose 800x600
2.Can you run a live cd, of any other distro on this machine. If so you could pop it in, copy the xorg.conf to a flash drive then use that configuration on your Vector set up.
it seems like it will not run 800x600 , since If I delete the "640x480" modes in xorg.conf , then if I try to run startX , I am told X cannot start..
You should not have to delete the "640x480" the xorg.conf will run the best resolution of the group by default. You need only delete the higher numbers. Also the lines for 8bit, 16bit and 24bit color should be exactly the same I believe.
xorg.conf is finiky, it will not start if you have accidently deleted or left in an extra " ie ""800x600" instead of "800x600"
Try idea 1. first this should rewrite your xorg, hopefully fix it, but at worst you will be back where you started.