OP--You have a WAY bigger issue than your GF possibly cheating on you in Vegas. That is your lack of solidity in yourself and in your life, no matter what happens. To be so devastated, at the idea, that something could happen, that you respond in fear and as if your world is already/ or going to implode from the inside-out, speaks volumes. It's a rough way to live.
I encourage to step back and look at YOU. Look at how "an idea" of something blows you completely off your center.
I understand the concerns about your girl with lots of men partying it up in Vegas. I get it.
The bigger issue isn't whether she cheats, or are you so fearful you break-up before she goes. The bigger issue is you have no foundation that supports you. Without one you feel powerless to her work itinerary, to the alcohol and men that will surround her, and to all that could possibly happen.
I am in no way suggesting a woman cheating on a man is not devastating, nor attempting to minimize that potential betrayal and pain. I am saying you need to have a more solid foundation in yourself and in your life, regardless if that were to ever happen. A man centered in himself, would not be scampering in anticipated pain. He'd know that if a woman stepped out on him that it would be the biggest regret of her life. He'd know that there are plenty of great women, that will happily snatch up the opportunity to be his new GF.
If you break-up before she goes, only do it to get yourself together, to work on you. Don't do it out of fear. Whatever you choose, focus on building your self esteem and your life so you know, without exception, that you are an amazing catch for a woman and that your life will move forward in great ways whether you have one riding with you, at the moment or not.
Step out of powerlessness and back into your power. Your life is your own. It will move forward in great ways, with or without her. She doesn't own your destiny. You do. Make it yours. You are the leader in your life.