Unfortunately, the nuclear bombs of the Great War, and the consequential fall out, to excuse the pun, decimated the original inhabitants of the area. That's not to say that the Yakima Nation is non-existent, but rather, that the area is is dominated by a multitude of peoples and races originating from other areas of the United States. The Yakima Republic's name is based entirely on the Old World county in which it is situated, as opposed to the Native American peoples that once dwelt there. We have super mutants, ghouls, robots and humans all living under the Republic's umbrella of racial diversity and special interest.

The whole appeal of the Republic, to the various peoples that form it, is that it accepts everyone so long as they can contribute to it, and die for it when the time calls for such actions. Such an appeal brings groups from far and wide. Imagine a land where you and your fellows could go and live, with relative security, so long as you paid your dues and accepted the established political system! It's a better alternative to the dubious NCR policies. 

If needs be, I can create the Republic's history so that you can all have a better understanding of how a collection of raider gangs, traders, tribals and vault dwellers formed a mutual security pact called the Yakima Grand Alliance, which then evolved into the Yakima Republic some decades later and sparked a civil war with elements of the Alliance not willing to lose their independence. 

It sounds better than some Native Americans somehow surviving two centuries of nuclear fallout, if you ask me.

But hey, don't let me tell you guys how it is. I just made the RP, what do I know? Excuse me whilst I go and invade someone else's' RP, and make an arse of myself for ... whatever reason I need to propel my inept soci... no, I'm sorry, I'm getting angry over nothing. Please, go ahead and stir the debate. I can take the Republic in a more Native American direction if you want, though I feel it's a bit late in the day for any major changes.