Hi, here's a solution that might work. It work for some guy on Twitter who told me so, I tried and worked for me too...
Anyway, go
here and download the 32-bit Media Creation Tool on your tablet. Now, I have a USB stick that has micro-USB so I was able to plug it into my VivoTab and create a Windows 10 installation USB.
I could have chosen to do the upgrade immediately (i.e. skip creating USB installation drive) but I tried and download was often broken then. So I made the USB.
I used that USB to upgrade Windows. It didn't complain about GMA. The display is working ok, it switches off completely (backlight too, unlike on preview builds, due to incompatible display drivers!) and brightness settings work as well (didn't on preview).
The only problem was... Broadcom wireless drivers were not installed properly! So no internet. I tried installing various drivers from other OEMs (but the same Broadcom chip). Nothing. Then I decided to uninstall the device and drivers using Device Manager (there was a huge exclamation mark there). Didn't bother with reboot, but rescanned for hardware changes from Device Manager. Boom, found Broadcom wireless SDIO adapter, installed the right drivers and gave me internet.
Are there problems? Yes. Sometimes there's text missing in Edge (in tabs and elsewhere). Text is completely missing from taskbar jumplists. Annoying. The dude from Twitter claims some text is missing from Cortana too. Other than that, it works ok, it's probably a few percent slower than on 8.1 but hey. It's barely noticeable. There is some stuttering when returning to Start Screen when in Tablet Mode, but it's minor and not a problem (for me at least).