I would be careful about how detailed a plan you develop for your friend. I know that kind of planning comes naturally to you, but that is exactly what can trip you up with a person like this. You might be doing it out of consideration, and maybe to assuage some guilt, but you run the very real risk that she will wonder why you are developing two plans when it would be just so much easier to hook into the one she knows you have for your family.
As others have said, a real friend would never put a friend in this kind of position. She probably doesn't know how to be a real friend, and when she can get her desperate hooks into someone willing to allow it (and she senses you will), that's what she's going to do. I'm not suggesting she's a bad person, just needy and boundary-less.
Bottom line: Is your friendship with this person more important to you than your family vacations? If it isn't, then how about trying whichever of the excellent suggestions people have made here that genuinely appeal to you (desire, not guilt), and then letting the chips fall where they may? Unless you allow this person to swallow you whole, you probably won't be able to make her completely happy no matter what you do.
And if you can't let go of the guilt, why not choose between guilts - focus on the guilt you would feel by making your own family trade their fun for hers. That strategy has worked for me when I was trying to wean myself away from bending too far backward for people who invariably turned out later not to be worth it.
Good luck!! I've been there, and can guess at the anxiety you might be feeling right now. Maybe it will help to know that one other person in this kind of situation always felt much better after not giving in.