I also have Android on eMMC of my VIM3 pro and use it very often but it doesn’t prevent from loading Twister. You can flash mainline Uboot on SPI in Krescue:
main-menu → Advanced → Experimental → Update/Write U-Boot by online to SPI-Flash
Krescue UPDATE
And switch KBI boot mode from eMMC to SPI either in Krescue again:
main-menu → KBI - Device Configure → Set Boot Mode to SPI
And you can load any os images from SDcard or USB without any issues.
Then if you need to load from eMMC while in SPI boot mode you can power on and make 3 short presses on Function key or
press Space to stop boot sequence and write:
kbi bootmode w emmc