By default, the unlocking options are hidden. You need to make them visible first.
Back on your PC (Command Prompt as Admin): unlock bootloader vivo y71 high quality
cd C:\adb
fastboot devices
(Should show your device ID)
Now, the critical command:
fastboot oem unlock
or (for some Y71 variants):
fastboot flashing unlock
Your phone will show a warning screen. Use Volume keys to select "UNLOCK THE BOOTLOADER" and press Power to confirm. By default, the unlocking options are hidden
Vivo does not allow unlocking via fastboot commands alone. You must use their official mechanism. (Should show your device ID) Now, the critical