No. Vivo Y81i does not have the fingerprint scanner but it does support the face unlock.
Vivo Y81i has FunTouch OS which is based on Android 8.1.
Yes, Vivo Y81i does have the OTG support.
No. Vivo Y81i does not have the removable battery.
You can record up to 1080p resolution videos on Vivo Y81i. You cannot record 4K videos via Vivo Y81i.