"Expanding Rapidly" does not mean that they necessarily annexed Vegas, characters in New Vegas refer to to the Mojave and surrounding environs as particularly dangerous compared to elsewhere in the world, and even if they did add Vegas, it's going to be a slog through Legion territory. I would guess that if they were expanding it would be into Oregon, Washington, Idaho, Montana, and maybe the Dakotas. That Wyoming/Montana region is likely scarcely populated anyway since (at least in our world) those states were sparsely populated to begin with which is why all the ICBMs were kept there.
Regardless though, considering that Fallout 1&2 had potential outcomes that were eventually declared canon and others that were declared not, it's not like these games are obligated to respect your choice in New Vegas. The NCR is one of the four potential outcomes, so if they declare it's that, that's what it is. It's not what I would have preferred, but I won't say that it's not one of the likelier outcomes. I would say that the NCR outcome is the most likely since it gives a "good guys win" scenario without everything ending up too rosy, and the Legion ending is the least likely since it's a "bad guys win" scenario that would require followup. The second most likely outcome is probably Independent Vegas since it's the most empowering for the player, and House comes in third.
