If you have tried all the methods above and are stuck, here is why:
If this option exists, your device will switch to the true Fastboot mode (usually a black screen with white text showing "Fastboot" at the top). If the option does not exist, your VIVO S1 does not have a user-accessible Fastboot mode. How to Open Fastboot Mode on VIVO S1
Most flashing commands will fail with:
: Hold the Power button for about 5 seconds and tap "Power off". If you have tried all the methods above
Now, type the command to reboot into Fastboot Mode: How to Open Fastboot Mode on VIVO S1