Again - I doubt the rental agreement will say that you can't take it to a neighboring state. There are a lot of people taking their rentals to California, Arizona (especially the Grand Canyon or Hoover Dam), or Utah. You might want to call once you have a reservation.
And Yosemite and SEKI take a long way to get to. The fastest way from Vegas would be through Death Valley. If you go that way, make sure you've got water just in case as well as a full tank before you enter. Other than that, the other way would be to go down.
As for rental tips, you're probably better off continually checking. As long as you don't prepay, most car rental reservations aren't secured with a credit card. I've even had multiple reservations open at the same time with the same rental company. Even if you don't show up, there's typically no consequence. The last time I went to Las Vegas, I started checking rates and reserved the first reasonable rate I could find. I kept on checking and checking, and made a new reservation when I found a better rate. In the end I got $84/week for a "midsize" starting Thanksgiving day - from Avis but booked on Priceline. What they had for us was a late model Hyundai Sonata, which was actually quite nice. We only rented for 5 days and the taxes and airport fees are based on exactly how many days you rent, even if it's a weekly rate. I think altogether it was $125.
One of my things is to check on Priceline and/or the other online travel agencies. Then once I find a good rate I'll check directly with the rental car company website. I've even found a better rate since they have some specials only on their own websites. I tried a July 28 to August 10 rental, and I couldn't find any decent rates except for the companies with questionable reputations for bad service and finding "damage" that could have been preexisting. Prices change a lot, so it helps to keep on checking as the rates rise and fall with the market.