Fiancee visas don't take 90 days, they take a lot longer than that. Although I have heard they are speeding them up a bit. Kind of expensive too and you have to do all the work in the US, so if you are something of a permanent resident in CR it would be tough to do. They go so out of their way to make your requests hard to process, asking for weird stuff and different documents all the time. I went through this horrendous mess getting my wife here legally and the waste of time and money is unbelievable. She went to a class to take the GED in Spanish. Out of 15 people in this class, guess how many were legally in the country? And out of that my wife was the only one still not working, because she actually was bothering to be a legal worker. The system is extremely messed up.
Don't even think of going through all that just for a trip to Vegas unless you think she will have reason to return quite a bit. It just isn't going to be worth the headache I don't think unless you think you might get married and would need her to get regular access. Even then, you cannot get married under a tourist visa, they will just say you did it under false pretenses and can deport your spouse with a 10-year ban on entering. Do lots of research on these things is my advice and double any amount of time you think it might take.