I am going home to KY for the Holidays Monday and my rx8 will remain in Vegas. My only question is, Do I need to worry about flat spots on my tire if it sets for approx 20-25 days?

Should be no problem but if you're worried just bump your tire pressure to maximum while the car sits and re-set the pressure when you return.

I think I would be more worried about the battery. Have read quite a few threads about that when sitting for long periods. I would do a search, but you might want to either disconnect it fully, or possibly put a trickle charger on it. Guess I would just disconnect it.