"a repair install will be necessary to get Vista/7 booting again after XP replaces it's boot loader"
What? Naaa you dont. maby the method you use. The easy way for me is pop in the vista disc (using xp) and use command prompt. then you do something called " DiskPart" Install on 2nd "part" that wammo its done. Learned it here.
Ok, so let me get this straight.
Somebody has Vista on their PC.
they install XP to a second partition.
XP overwrites Vista's boot sector and boot loader with it's own. Since it doesn't recognize what is already present, it does <not> setup a dual boot configuration and always boots into XP.
From within XP, you put in the Vista disk, repartition your drive (using diskpart for no reason), and reinstall Vista to a new partition.
What you describe sounds more like you are describing what you do to install Vista <AFTER> XP. I was clearly referring to circumstances where XP was installed <AFTER> Vista.