I agree with those that said you should pay her a 1/2 salary while you are gone, since this is a regular "job" for her, and not just a one off, every once in a while babysitting job. And you will be gone for quite a while, so... it's really not fair for her to lose on that that much income for something that is out of her control. Plus since she seems to be very good, you really should treat her accordingly.
If, however, she chooses to go on vacation, I would not pay her. You should probably let her know that now to set the ground rules (as well as let her know that if you are gone for more than a week, you will pay her 1/2 the average of what she would received for that amount of time, or whatever it is that you decide).