Listen to the wisdom of Toots!
Let me elaborate on that wisdom:
The problem with telling her "July works best for us as VorGuy has summer camps in June and August" is that you are not keeping your
vacation and their
vacation separate. That plays into her hands; that gives her the opening to keep pressing for you to vacation in June, because, of course, you all will be vacationing together!
You need to make it clear that, whatever their
vacation plans are, yours
are to vacation in July. Don't mention anything else, not even "You are welcome to come in July."
Figure out a single sentence, and turn it into a "Toots special." When she tires of getting the exact same answer every time, she'll stop hinting/asking.
May I suggest, "That's nice; we'll be vacationing in July, as always." Then beandip.