If MS doesn't want you to change the sound on vista, they won't let you. Remember, this is the palladiumified windows. You don't even have root on it. I have no difficulty believing that MS would use their DRM infrastructure to keep people from changing this wav file.
Even if they didn't, it will likely be registered as in use by the winlogon process and unable to be changed unless you mount the partition under a separate OS.
Then again, they may decode to be total dicks about it and hardcode the sound into the software (no wav file to change).
Remember this is MS we are talking about. "Where do you want to go today (as long as it's ok with us and our REAL customers, the RIAA and MPAA)?"