I would support you:
not sure if i reached fastboot mode, because it looks different for me than other phones (and the difference is not chinese

)
1. phone is in normal booted mode, pluged in via edl cable (null cable), oem unlock in dev options enabled
2. powershell: adb reboot bootloader (I thought this would be wrong, but this was the first command in a youtube video for unlocking vivo phones/same vivo compiled adb/fastboot files as here posted for download)
2. (optional?) powershell: fastboot bbk unlock_vivo --> Failed (remote: unclock fail.) (yes: unclock^^)
but the phone booted after the "reboot bootloader" showing the following: Fastboot (blablabla Chinese), more Chinese, green Button (translated in english: boot system) and black button (translated: recovery).
3. step by step aftool via video tut
a) selecting India as default country (in unzipped firmware folder you are going to find the countries: hk, cn, in, ru etc.)
b) checking if all files are checked and looks valid
c) hitting "Download" Button
Error message:
"This mobile phone brush authorization failure, please try again after recovery mode clean data, or use 9008 upgrade"
Info message:
"Download terminated! To download again, you need to disconnect your phone from your computer....."
Ok, some questions:
I. what is meant with "please try again after recovery mode clean data"?
II. is the firmware provided in 1st post usable for 9008 upgrade method?
III. Are you aware of some malware detection because of aftool auth-exe? maybe false positive...
some infos about my environment:
using win10 64bit via virtualbox with usb3 extension package.
the passthrough of usb (phone and fastboot) needs some work:
after I installed the adb-driver, vivo-driver and qfil-driver the phone is recognized in the virtual machine, in virtualbox you should do a fix mapping of the "nex" for the vm/guest. after the phone reached the "fastboot mode" (I am not sure about the right fastboot mode) the device would not be recognized anymore in the vm/guest. you should use a usb-debug tool like "USB View" to investigate the vendor and product IDs (hex-codes) of the "android adb interface" when the phone is in "fastboot". with these info you could bind the usb-device to the vm. voila the phone automatically jumps into the vm/guest.
Only in case that you are concerned about the malware detection, too.