Some good points are raised here, sure, but I can't help but point out one little problem. Legion with its relentless conquest and promises of harsh, but fair frontier justice; Mr. House with his little city with robots; all of this looks plausible and possible in a post-apocalyptic environment, but the story of New Vegas takes place two fucking hundred years after the war, 204 to be exact! In our world, 204 years ago Napoleon was still alive and infantry still used muzzle-loading flintlocks.
For comparison, Fallout 1 takes place eighty or something years after the war: all big cities lay in ruins, strange beasts ravage the wasteland, no means of production - and thus people are trying to survive and recreate civilization with what they have. Fallout 2 already shows us some progress: San Fran shows very little signs of past war, NCR is clean and prosperous, and some small cities emerged quite successfully, like Vault City or Redding.
40 years forward, and the situation is even worse than in Fallout 1: people still live in ruins, rubble and wreckage is everywhere, and progress is really nowhere to be seen. Yeah, I know that Shittesda is the one to blame for this bullshit, but New Vegas suffers from this as well, inheriting this misunderstanding like AIDS from its parent.
What I'm trying to say is that Legion would have looked perfectly viable in the immediate years after the war, not two centuries past. A theoretical setting sixty years after the war, where some confederation of small settlements is confronted by Legion and each side has its points; Mr.House little paradise not touched by the war, but with obvious implication that it won't last long in the current environment... Moving series so far in the future was a mistake, but alas, I guess again that it was one of Shittesda requests, to sandwich New Vegas between Fagout 3 and Shittesda next game.
Both in the pretty same time period.