I haven't been to Vegas so I'm not sure about that. My advise for extended plane rides is to have tons of small activities. Also, save the activities until right before she starts to lose interest in the plane. The first 20 minutes she might be really interested in just looking around or out the window. Show her some of the plane and equipment. Then do a couple of activities like reading, coloring. Then have a small snack. Then show a movie.For the last leg of the plane ride, I would have some activities that she hasn't done before. They have these aquadoodle and portable doodle pads. I also got a real wallet and put real looking - either old or from junk mail credit cards and business cards, and I got a bunch of old keys for her to play with.
For the plane ride I would make sure I had plenty of food, drink ( especially just in case of extended time on runway). I would bring something like candycane or lollipop or some other treat to use as an emergency if things get really bad. My daughter also used a binky for sleeping and because of the ear popping I brought that just in case. I'm not sure what else you can do for popping other than chewing gum. (My husband gets really bad pain during takeoff and descent which causes his ears to get permantently clogged.)
Good luck and have fun.