Before vivo, I had an intex aqua power (mt6592). It did have fastboot, but couldn't be accessed with button press. Command from terminal worked to boot in fastboot mode.
If you can enter fastboot by connecting to pc and running adb reboot fastboot, please try to run fastboot oem unlock(will wipe data if it really unlocks the bootloader).
adb reboot bootloader
Not worked. I think. They somehow managed to prevent booting to bootloader at any manner