You're using the wrong image. You need to download images that have S812 in their name.
https://yadi.sk/d/CxRZ4LFOuHcJW
Armbian for S812
https://yadi.sk/d/QCGU0PIv3G5kEa
Yes, the system tries to use the embedded dtb (from the eMMC), but this only works if the eMMC uses a the same kernel . For example, if the eMMC installed has a version of LE that uses a similar kernel or Armbian\Linux.
Version 20180213.
Updated part modules of WiFi + updated firmware. Tested Wi-Fi in s905 (Vega95 Telos\Pro).
Changed the mt7601u driver.
Added deb packages from the latest version 5.41. Please note that I recommend using firmware-aml-20180213.deb, if WiFi will not work (after downloading the desired module manually), you can try using the old version of firmware-aml.deb
Try to install additional packages from the firmware.
https://yadi.sk/d/VrXItQ-k3SKEth
Since the beginning of the test, if it works, then it will be "theory".
Try the version from the catalog on the website mali6\5.41. Try to run from external media and test the mt7601. If it works, you can try to build a separate deb package for legacy images.
1. Try updating the multi-boot using only the image Armbian. To capture the image, copy and rename the dtb file, to run Android, to run multi-boot activation.
2. What firmware version is installed in the eMMC of your VIM2 ?
I have this image running on VIM1 (s905x) and VIM2 (s912). What u-boot is installed ? To run the kernel 4.9.40 on VIM1 (s905x) need a new u-boot. I use my version of u-boot with built-in multi-boot.
To run from external media, you can use any option of the dtb. The correct option dtb important to install the system in eMMC. Then you need to use strictly correct dtb.
It is better to use staff team sdc_update. It is better to know the features of the installation of the new u-boot (data entry, calculation of checksums, etc.).