I was in the same boat, I wanted to do a complete clean install of CE to the eMMC of my VIM3 but ceemmc kept on throwing errors.
I burned all Android firmwares one by one (newest to oldest) and tried to install to eMMC using ceemmc from CE20.3 after every burn. These are firmwares I tried:
- vim3-android-9-32bit-v231122
- vim3-android-9-32bit-v230720
- vim3-android-9-32bit-v221228
- VIM3_Pie_V210908
- VIM3_Pie_V210128
- VIM3_Pie_V201112
- VIM3_Pie_V200917
After all these attempts I decided to try booting CE19.5 from SD and run ceemmc from there, and lo and behold it would install just fine.
As a test I burned the latest Android firmware again (vim3-android-9-32bit-v231122) and tried to write CE to eMMC again using CE19.5 from SD and ceemmc would still install to eMMC just fine. After that I did a manual upgrade from the CE menu inside Kodi to get to CE20.3.
So my conclusion: the issue is not with the Android firmware on the eMMC, but something in CE20 broke the ceemmc tool. Note: I also tested CE21-Amlogic-no and it has the same issue as CE20.
TL;DR
In order to use the ceemmc tool on a VIM3 you need to use CE19.5 (you can burn the latest 32bit Android firmware though).