I don't know if this will help and I don't remember if it worked with Gslapt when I installed VL6 Standard Deluxe many months ago. But this is what I found in my /etc/slapt-get/slapt-getrc file:
My guess is that you'd change that to
Put your CD in the drive and mount it. See if you get a directory in /media/cdrom0. If it automounts to /mnt/cdrom, unmount it as root and mount it at /media/cdrom0. Then run Gslapt and see if this source is listed in Edit menu, Preferences, Sources. Make sure it is checked. You have to run Update in Gslapt. If this doesn't work, try again with the CD in the drive but unmounted.
If no luck, do the same steps again but this time make the change in /etc/slapt-get/slapt-getrc to SOURCE=file:///mnt/cdrom/packages
Put a # in front of SOURCE=file:///media/cdrom0/packages to comment it out so it won't appear in the Sources list in Gslapt. run Gslapt as above, Update, and see if you have any luck this time.
I don't know enough about this to know whether my suggestions will do any good. I do remember that I had a hard time getting the right syntax for adding the CD to the Sources list so that it actually worked.
Let us know if either of these helps you out.