Interestingly people have been given different reasons by support why the Halo 2 Vista MP is down. Some supporters say due to maintenance, others because of an update. At least none of them said that the servers have been just shut down for ever.
But it is very suspicious, that this downtime is now several days. Well, as far as I understood, the servers can be shut down by MS sometime during October 2012 (because of their Microsoft Support Lifecycle Policy.)
At least it says that “mainstream support” for Halo 2 Vista expires at 09.10.2012 (european date format).
So as far as I get it, the MP servers should still be on at least until that date, but that´s just an assumption.
I can’t imagine microsoft keeping the servers alive for longer than they have too due to their policy; I should re-install H2V and go finish my multiplayer achievements before they become impossible when the maintenance is finished.