I believe they never expanded in to the northern wastes. New Reno is not a state and Vault City was said to be independent.
Vault City was independent and New Reno was not a state but NCR did take them over at some point. FO2 NCR was nothing but a much more improved Shady sandsv (areas around it).Chosen One helped them become the NCR we see in New Vegas. New Vegas makes it clear that NCR will annex or invade anything they want. "The expanding Republic considered Vault City a fantastic candidate for inclusion into its growing organism, however, offers of joining were repeatedly declined, with Vault City preferring to stay independent. With no progress being made on the diplomatic side, the NCR Congress approved a secret initiative intended to force Vault City to join."
It seems very likely that they did. All the towns and farms in the North. NCR sends troops East across the Mojave all the way to Hoover Dam but they don't think to go north along well established trading routes?
Edit by "nothing but Shady Sands" I mean on the FO2 map. NCR had taken over eveything south of them that is not on the map. FO1 locations like LA and the Hub.