I suppose you could use Vista's option in Disk Management to "Shrink Volume" in order to create enough space on the hard drive for your second installation and then put a second copy of Vista in there. You will need a proper Vista DVD (not a manufacturer's recovery disk) to do this.
As the hardware is the same, the second copy should "Activate" OK with Micro$tuff again.
I don't have a fixed hard drive in my PC, I have a hard drive caddy and can slot in whichever drive I want to - XP Home, XP Pro, Vista. I have two drives with Vista on and this was achieved by using Acronis True Image to backup the first drive to an external USB hard drive and then restoring it to a second hard drive. If you did something like that, you could have your two Vistas configured differently.