Although I would be inclined to stop when XP was installed and not re-install Vista. Is this possible when the Vista files are already sitting on the hard drive?
Yes - just remove any existing partition and re-format, then install XP. No Vista.
And why aren't the Vista files deleted when the hard drive is re-formatted?
They would be.
A dual-boot requires a 2nd partition for the 2nd OS.
It may be possible to install multiple OSs on the same partition, perhaps someone else can advise?
But it IS possible to install XP AFTER Vista - see
here for details. You will have to repair the Vista bootloader after installing XP, however.
SD