You can start by spicing it up at home with a few swinger videos, if you are not a swinger yet. Since you have a 3-month head start, you can warm her up to the idea, and even get to some swinging events together. That will loosen her up to want to try what you are suggesting while on the trip to Vegas with her friends.
Who knows... you might help recruit another of her friends to start swinging, too. The more, the merrier. And you will probably benefit from that, too. Happy trails...
