What about agreeing ahead of time on some things and let other things be left to decide once you are on vacation. For example, agree to give her 2 of the 7 nights you are there (assuming you are there for one week) off, but agree that it can't be the night before you leave, or whatever. Also maybe agree that she has one full afternoon or day to herself, or however you would want to work that.
As for the pay, I would think sticking with the regular rate you pay her would be ok. Any extra time would be compensated for by covering the cost of food and travel, so I would think it would even out.
I think the most important thing is to draw up your plan, or at least write down your ideas, and make sure you and your husband agree on them, then present them to your nanny. Maybe she has a small request for the trip, and it's something you could work into your plan.
Hope these ideas helped!