I know that sometimes it is nice to just want to say **** off! To people so I will totally validate you in your anger. Why choose LV in August? Why not consider the fact that Eliza will be miserable? Why not spend more time involved in your lives so they
know that she won't nap instead of you having to be the petulant child and bring it up.
Here's the good news: (!!)
August is far enough away that no matter what you don't know for *sure* what sleeping pattens will be
There are so many darn people in Vegas that there must be something amazing for a mommy and cutie patootie two year old to do together that would be amazing and memorable and totally fun for both of you
You have time to plan! Or plot a scheme

. Self fulfilling prophecies, baby! Try not the believe you will hate it because if you believe it, I will happen. Focus on a few positive things and try to remember that for some reason "family dynamics" is synonymous with "bugs me most if the time but I love them"
I do agree with N in that if this will cost you $$... Just say no.
Good luck