I agree, you should go ahead and send the invites now. I'm having a vow renewal next summer too, July at CSA, and I sent out my invites in June! The more time you give your gurests to plan, the more likely they will actually be able to come! We have invited 6 couples, and so far 3 are positive, 2 are possible, and only 1 couple can't go. One of our couples already has reservations (like we do)!
I was thinking that if I send out the save the dates now with my wedding site that has info on Couples and thier website everyone would have time to plan and book. My dad booked right away to get the $300 resort cerdit deal, lol.