That's a sticky situation...Are her parents religious or something? Is she a Virgin?

I understand that if she lives at home, that overall her parents can still tell her what to do, but they have to understand that she is a 21 year old woman! But, until she moves out...they can ultimatly tell her what to do. In this case, I think it would be a bad idea to talk to her parents. They may hit you with..."why is it that you want our daughter on vacation with you?" or something to that effect. They may think you have bad intentions...
I would go with your original plan. I mean she has to have another g/f that can cover for her...