I had a similar situation (fake/vendor rom in a Redmi Pro from Banggood).
I flashed the official stable 8.1.2.0 rom via fastboot. It turned out that MIUI 8.1 has a new feature. In the developer options section of the settings you can check if the bootloader is locked or not. Mine was already unlocked so i just flashed TWRP in fastboot and from there MIUI.eu rom.