Update...Thanks go to the concierge folks here at Lagunamar. It's the best concierge service we have found anywhere. They put me though to a knowledgable man in Orlando, who got us onto the right track.
It turns out that the Vistana web page is broken. It should have been letting us book for Jan 2023. The man in Orlando did the booking for us...for the first 13 nights of our planned stay. I am to call back tomorrow to book the remaining 13 nights.
So Aventuras availability was never the issue.
FWIW, I also received an email from Vistana that sheds added light on the issue. It seems that no one is allowed to use home options for anything longer than 2 week stays. Hence the need to make 2 bookings for longer stays.
I hope this info helps others.
By the way, Lagunamar treated us and 15 other owners to a very tasty 4-course dinner with wine pairings at La Cenacola on Tuesday evening. You might keep your eyes and ears open for this if you stay here.