Warning: this will only work with the older firmware (Android 6.0.1/1BEBFDB1 )
What this will achieve:
You will flash a modified boot image that runs selinux in permissive mode.
You will flash a custom recovery that allows you to root the device.
After you are done, your userdata will most likely not be encrypted anymore. You can re-enable encryption in the system settings.
1. Install the usb drivers that come with the device, in /system/etc/USBDriver.iso.
They are also in the download package, in USBDriver/.
Also install SUGAR QCT_SP v11.1.2 Setup.exe from the download package.
2. Connect your device to your computer and boot it into EDL mode (enable usb debugging).
adb.exe reboot edl
3. Run SUGAR QCT_SP (use credentials from download package), select VF1400 device and click on the upgrade button.
It will download the latest official firmware update into "C:\Program Files (x86)\SUGAR QCT_SP_Gotu2\bin\VF1400-2AVXXXX" (VF1400-2AVDDE1, for me).
After the download is done and the flashing started, make a copy of the VF1400-2AVXXXX folder to your Desktop.
4. While the flashing is going on, we can start to modify out copy of VF1400-2AVXXXX.
In it should be a folder, most likely called 1BEBFDB1 (version number of the update).
Inside that folder should be the .mbn files we care about: B1BEB0B0CD00.mbn (boot image) and R1BEB0B0CD00.mbn (recovery image).
Possibly they will be called differently, but I assume the boot image always starts with B and the recovery with R.
You can check "C:\Program Files (x86)\SUGAR QCT_SP_Gotu2\tool\logs\Sugar_XXXXXXXX.log" to make sure which file is which.
Replace B1BEB0B0CD00.mbn with boot.img from the download and R1BEB0B0CD00.mbn with recovery.img.
5. After the flash is done and the device rebooted, put it in edl mode again (after enabling usb debugging).
adb.exe reboot edl
6. Now, *before* we start SUGAR QCT_SP again, copy the VF1400-2AVXXXX into "C:\Program Files (x86)\SUGAR QCT_SP_Gotu2\bin"
7. Start SUGAR QCT_SP, select VF1400 device and hit upgrade.
Congratulations! After flashing and after the next reboot you are running with selinux in permissive mode and you can flash SuperSU from the recovery.
Now copy the latest SuperSU flashable zip into your internal storage and install it from recovery.
The recovery used is CM-13.0, device tree based on https://github.com/zwliew/android_device_xiaomi_land
Being compiled with prebuilt kernel from boot.img.