If you manage to unlock the bootloader using unofficial third-party tools (like UnlockTool ), you might be able to flash a GSI ROM since the device supports Project Treble.
Most "ROMs" found online for this device are actually Official Stock ROMs used for unbricking or fixing software issues, rather than custom performance-enhancing firmware. Risks and Warnings
Attempting to force a custom ROM onto a Vivo Y1s comes with significant risks: