As of October 2015, the master server list, which is required for Live multiplayer, is down, and there's no official news from Microsoft or 343 Industries, so it's possible that Halo 2 Vista multiplayer is permanently dead.
It's still possible to unlock this achievement though, and I'm posting this solution because the other one on this site is missing an important caveat.
You must complete the campaign on Legendary first. Keep in mind that simply having Legend on your gamercard is insufficient; your Legendary campaign save file must be on the computer you're attempting Hired Gun on.
Start up a solo multiplayer game on the map Uplift by going to Live > Custom Games. I've heard that it does not work if you go under Network (LAN) mode, but I haven't tried that myself. Once the game starts, go to the large orange/red/pink-ish beam in the center of the map and jump into it. It will register as a suicide, and Hired Gun will pop.