This is bad. This is really bad, man. She's looking him up on Facebook. The only reason to do that is because she wants to know more about him. And it is 100% possible that she's Facebook messaging him and then deleting the messages. It's 100% possible that she's keeping in touch with him and that some part of her wants to meet up with him the next time she's in Vegas.
Get it over with. Tell her you need to talk to her about her trips to Vegas because you know she kissed a man there and you know she's contacted him on Facebook. Bam, just lay it out there. See what her reaction is to saying you know "she's contacted him". But regardless, you have to stand up for yourself and for your marriage. And you have to start admitting to yourself that there could be more to this than you realize. You're convincing yourself that she didn't do anything or she'd have told her friends about it. Well, just because you didn't see a text doesn't mean there's not more to the story - and those additional details could have been an actual phone call.
This is your one chance to confront her before this gets worse. Confront her now or face the very real possibility that this is the moment you lost your marriage. If you don't confront her about this then next time she goes to Vegas BAD THINGS could happen and you might not be so LUCKY to see the texts next time.
Be smart about this. Confront her.
And, last but not least - NO MORE TRIPS TO VEGAS. Seriously. She CHEATED on you. No more Vegas. If she doesn't agree to that then you have to be prepared to leave. If she's willing to prioritize a trip to Vegas above repairing the damage she's done to her marriage then YOU NEED TO RECOGNIZE THAT. You deserve better than that! Every loyal spouse deserves a loyal spouse - understand? So, you have to confront her, and you have to tell her that she can't go to Vegas anymore or the marriage is destroyed.
This is going to take courage and backbone, OP, but you have to do this. Confront her now or your marriage is at very high risk. She cheated already and is keeping tabs on the guy she cheated on you with. This is extremely serious.