Well, basically the titel of the thread says it all. The Halo 2 Vista online multiplayer is back online and working.
See this thread:
http://forums.xbox.com/gfw/technical_support/connection_networking/f/265/t/177955.aspx
Also, I just went online and tried it out myself. It works.
Because a lot of people were asking during the last week, I thought that a new topic for this info would be appropriate, so that everyone who looks in this subforum will see it at once.