If that's the case, just partition a new operating system onto the hard drive if you really want to keep Vista (although, to my knowledge, Windows XP for 2.3 servers was superior, then Windows 7+ for anything later than that was vastly superior, due to having Windows Aero) I had vista and ditched it the moment I could, just a terrible operating system.
GW2 is not a heavy game, fyi. Cartoony styled graphics MMO's can be ran on near enough anything. But, I digress, if he's just going to play Mir, don't bother upgrading.