If you need to repair your VIVO S1, it is safer to use (Power + Volume Up) to wipe data or apply official OTA updates rather than Fastboot.
This is the standard method for entering Fastboot Mode when the phone is powered off. Since the VIVO S1 does not have removable batteries, you must follow the timing precisely. How to Open Fastboot Mode on VIVO S1
If your buttons are damaged, you can use a computer with ADB (Android Debug Bridge) installed. Enable in your phone's Developer Options. Connect your VIVO S1 to the PC via a USB cable. If you need to repair your VIVO S1,
: Once completely off, press and hold the Power button and the Volume Up button simultaneously. How to Open Fastboot Mode on VIVO S1