Dirty flash over phh AOSP. Many people had same problem with RR. Flash phh v109. Boot to system. Flash TWRP, reboot too recovery and then flash System of Viper to System Image via TWRP. After that, reboot. Many people got RR working this way and this should work the same with ViperOS
You need an unlocked Bootloader and EMUI 8 (or 9 if it available).