[Jeebus]

First of all, mr. paladino is correct.
Second there are three things to consider.
1.(probably the most important) if you don't ask her out now what is the chance she is going to meet another [censored] buddy or something else and not really care in a month.
2.Do you personally feel like you would be a dick, or in another word, do you think you would fully enjoy yourself when getting some vegas ass.
3.Do you care that she may get some ass while your in vegas?

The first question obviously has the most effect because if you want to stay with her you may have to ask her and then can't get some ass. Otherwise she is obviously cool with the idea of hooking up just to get some, so you can't be faulted for doing it. The dick move would be to pick up the clap in the process and give it to her.
If you at all feel like you would be a dick don't do it, you might get odd remorse issues and [censored].