I'm thinking about getting Fallout New Vegas for PC, but both my brother and I will want to play it on our own Gamertags and earn our own achievements. Is that possible with one installation of the game?
Fallout New Vegas is not a GFWL game, it's a Steamworks game. You cannot play the game from different Steam accounts, even if the game is installed in the same computer. Each account has to have a different serial number associated to it, so you have to buy it twice, one for you and one for your brother. The same goes for Fallout 3, which is a GFWL game.