I don't know about BCV, but I was able to get a studio at VWL 5 months out this year and at 3 months out, was able to get a studio AKV - Jambo Savannah view.
FWIW--I booked memorial day at the 7 month mark for beach club and got in no problem. I added an extra night on the end no problem 2 months ago. Had no luck in adding the Thursday night 2 weeks ago and waitlist failed (didn't realize I had a limit of 2 per year, it wasn't that critical!). So we are staying at OKW the first night and then 4 nights at BCV.
Probably depends on how many owners want to stay there at that time I suppose.
I would definitely call right at the 7 month window and be prepared possibly to not be able to book the entire stay at the BCV. Three weeks is a long stay to be able to get all your nights, you might find there is simply one night that is already booked up.
Historically, had you called exactly 7 months out, you would have no problems booking BCV for August. No guarantee that will always be the case in the future.
Not sure about BCV, but we made ressies (3 rooms) just under 7 months for our Dec trip to Kidani (2 bedroom savannah view, 2 bedroom standard view, studio standard). Best of luck to you!!