Here is what I did to install the latest SOHO ISO. I started with four partitions: HDA1 was my Slackware partition which I was running as I performed all of this; HDA2 was the partition to which I was installing Vector; HDA3 was a data partition which was completely ignored; and HDA4 was my swap partition.
First we need to extract the 'initrd' from the ISO.
mount VL5.8-SOHO-final.iso VL-MOUNT/ -o loop -v
cp -v VL-MOUNT/isolinux/initrd.img initrd.img.gz
The above code will mount the ISO as a "loop" device. This means that, once mounted, you can access the contents of the ISO just as though it were a disk drive. "VL-MOUNT" is just an arbitrary name for the mount point. The file, "initrd.img" on the ISO is actually a GZIPped file, so we unzip it after we have extracted it from the ISO. After the file is extracted, I unmounted the VL-MOUNT directory so that I could re-use it (alternately, I could have used a different mount point for the following commands).
Now we are going to mount the extracted 'initrd' filesystem (again, as a loop device) and mirror some of the current system directories so they will be available to the "setup" program later on.
mount initrd.img VL-MOUNT/ -o loop -v
mount /proc VL-MOUNT/proc/ -o bind -v
mount /dev VL-MOUNT/dev/ -o bind -v
mount /sys VL-MOUNT/sys/ -o bind -v
We are now going to mount the original ISO onto the initrd filesystem so that the setup program will be able to access the files on it.
mount VL5.8-SOHO-final.iso VL-MOUNT/mnt/source/ -o loop -v
Next, we must prepare a mount table for the initrd so that it knows where the active root system is (if you wished, you could use a text editor to create the file). The "/dev/hda1" is the system root partition at the time you perform these commands.
echo "/dev/hda1 / auto defaults 0 1" >VL-MOUNT/etc/mtab
echo "none /proc proc defaults 0 0" >>VL-MOUNT/etc/mtab
Everything is ready to 'chroot' to the initrd directory and perform the setup. HDA1 and HDA3 are excluded from the setup so that they are not accidentally destroyed.
EXCLUDE_PARTITIONS="/dev/hda1 /dev/hda3" chroot VL-MOUNT/ /usr/sbin/setup --hosted