So I killed Caesar, Vulpes, and Lucius all at once while playing New Vegas. Now that the three main figures of Caesar's Legion are dead, who is the leader? Will the game ever bring up another figure? I haven't finished the main story-line so I have no idea if killing him affects it. Does it? I'm fighting for NCR.

Legate Lanius, the Monster of the East and Caesar's right hand man, becomes the leader but he does not appear in game until the Second Battle for Hoover Dam.

Also, talking to Boone (I do believe it was) reveals that he had intel that the Legion has a long list even beyond the legate in case any power get disrupted they can just get the next one in line.

I do believe it was Boon that said it

i'm pretty sure too, either that or the leader of the NCR Rangers.

The Chief? Possibly so

Eddie is a touch loco for bringing all his top leaders into one battle (including himself who has no reason what so ever to be there personally...). However he is not crazy enough to bring all of his commanders.

Joshua appears from the darkness, takes control of what is left of the legion and brings them all to Zion. Once they get there he loads them up with mind altering drugs which turns them into peace loving hippies, all they do now is dance around a fire singing Kum bye Ah my lord....

....Damn you... Now I have to kill Graham again. (4 times wasn't enough)

Much like Rome in the late republic through the empire, whoever controls the loyalty of the legion (so Lanius, who knows who after he kicks the bucket) gets the throne.