I am not sure if all versions of Vista supported mirroring, as I haven't been using Vista for years, but I usually create a mirror, break it and remove the old drive. That's the easiest, and actually you can work while the mirror is being generated, so the whole disk upgrade takes possibly 5-10 mins of down time including opening and closing the case, and reboots.

You may probably upgrade to 7, since you're at it? I'd recommend it. I am not a Microsot, or Windows, or Vista hater :), I just think you'd benefit from a newer, faster, safer and more recent OS, extremely similar to Vista as to GUI. You won't have to re-learn anything.