We booked about 10 months in advance. You probably don't need to book that far in advance, but we were visiting several other places in FP as well as setting up a family gathering in Hawaii. So, we thought it best to have plenty of time to make changes, arrange flights, and so forth. You can contact the Vahine Island Resort (Elisabeth or Landry) and get a sense of how busy they might be during the time you plan to visit. Vahine Island is a rare and beautiful place -- definitely worth a visit. [Sorry but TripAdvisor won't let me post the manager's e-mail addresses ... you can contact them through their website, however.]
Get an answer from Vahine Island - Private Island Resort & Spa staff and past guests.