Vivo devices with Android 8.0 or higher often have "Anti-Rollback" protections, meaning you generally cannot flash an older firmware version than what is currently installed.
Vivo_1801_MT6762_8.1.0_UMT.7z File Size: ~2.1 GB Scatter Compatible: MT6762 Vivo 1801 Flash File Umt
Locate the (usually two small gold dots near the battery connector). Vivo devices with Android 8