December UY here. Kind of in the same boat, trying to book a split-stay at VGF and BCV, and yes I feel the frustration.
I told myself I will never do this again, but last year I began a walk at the end of November as I heard availability would be an issue in early December at both resorts (was our first time trying to book in December). I guess you could say I "walked" across UYs, in which I had to call MS in order to have a reservation crossing UYs (Nov 29th-Dec 3rd). I had MS simultaneously bank 2024 points into my December 2025 UY to make sure those ('24) points were used for the reservation. Then, once I wanted to move my reservation forward (completely) into December, I called MS to do this once again, banking even more '24 points into December 2025 UY in order to use those 2024 banked pts on the reservation.
All this to say, I WAS successful, BUT it ended up being quite the issue later on....
A few months down the line we ended up changing our GF reservation down to 3 nights instead of 4, and that screwed up our entire points allocation. The system returned a lot of points back into our 2024 UY (even though I already had banked them all) and it was a whole debacle in which I spent around 10 hours on the phone with MS over a series of like 8 calls in order to get the whole thing fixed. I was basically told I probably wouldn't have had the whole issue had I not began that reservation in November and moved it into December.
The hassle wasn't worth it to even try that again for this upcoming year, so as of now I have GF preferred view resort studio booked for this coming December and a 1BR at BCV, and I'm ok with overspending on points a bit compared to what I went through this past year. I have learned my lesson lol.