As far as I know, there is not an actual, bonified street date for the release of Windows Vista OS. It probably will not be out until the end of 2006 and possibly 2007.
Given this, it is quite possible that the name Vista might be changed to something else. This wouldn't be the first time it happened with a product and wouldn't be the last.
Technically, Vista is the first market name MS gave the future OS. (Longhorn was merely a code name that was never intended to reach the public, but these code names usually do anyway.) It is very possibly Vista will not be the actual market name chosen once the OS is actually released.
Of course, it is also possible they would never change it again and Vista WILL be the market name upon release of the OS.
However, you have to think, why do you think it's worth more than mid $X,XXX.00? Do you have a legit reason to believe this or is this mere opinion as is with most domain appraisals/speculations?
Is this buyer really legit anyway? As far as I know, it costs the buyer nothing to contact you and have the transaction completed via SEDO. Why would they just contact you directly? How would the buyer benefit from that seeing how SEDO has escrow service which is free to the buyer and helps protect the buyer?
Lastly, greed always loses out in the end despite what the successfully greedy say. Greed breeds more greed and then turns into a vicious cycle. Something to keep in mind.
Good luck.
PS. Are you not concerned that the buyer is watching this thread. I know if I was thinking of buying a domain for that much, I would put a Google Alert out for it in the meantime.
PPS. Why did you reg this domain in May? Was there another reason for the name or do you have some insider info on MS products? As far as I know, the release of the name Vista was not made public before July 2005.