There might be a way to fix this by having GameRanger (a company that provides free multiplayer servers for online games) take over the Halo 2 PC servers, just like it did with Halo 1 PC servers. if you want, you can get halo 1 pc and play as many games online as you want for free using GameRanger. I'm sure Microsoft would be fine with having GameRanger take over the servers for halo 2 vista, all we need is like a petition to get people to sign off and then send it to 343 and Microsoft and GameRanger (http://www.gameranger.com/)
update: also if you don't want to purchase halo 1 pc, you can download the trial version for free and play halo 1 PC Demo for free on gameranger (its same thing as the full version except you only get to play on one map: blood gulch)
if you want a legit PC copy that you should be able to use Project cartographer with, grab a used copy on amazon
click on used and order one from there