Unlike standard Android devices, the command fastboot oem unlock does exist on the Vivo V9.

Technically known as prog_emmc_firehose_8953_ddr.mbn , this file is used by service tools to bypass the bootloader. Since the Vivo V9 uses the chipset, the loader serves as a bridge for advanced maintenance tasks. Primary Uses

Unlocking a bootloader removes the verified boot process. This can make the device more susceptible to malware and unauthorized access, as the security layers designed by the manufacturer are disabled. Data Loss: