same here. Haven't heard of that sort of downgrading. Either way I just got a new laptop and instead of downgrading to XP (like it better, more compatible), I did a partition for XP and left my Vista OS.
Eventually Vista will surpass XP, in everything, specially looks (which it has surpassed already).
If you are sure about this "downgrading" then go ahead and do it; otherwise I'ld advise to do a partition and leave your Vista there, you might need it for later; unless you have the cd key and not much free space in Hard drive.