In order to boot Armbian from VIM3 with pre-flashed OS on eMMC you need to:
- Download Khadas Rescue tool for VIM3 and flash it to a spare SD card
- Insert SD card, attach USB keyboard + monitor or a serial console and boot into Khadas Rescue Tool
- Choose: Advanced -> eMMC -> Fast erase and Advanced -> Erase SPI Flash (Yes). Go back and select "power off"
Now you can boot Armbian from SD card and install to eMMC the standard way.