Vivo Y81i comes with 5V / 1A charger. It does not support the fast charging.
No. Vivo Y81i does not have the front camera flash.
Yes, Vivo Y81i does have the OTG support.
Vivo Y81i is made of Aluminum Alloy and industrial grade polycarbonate material.
Yes, Vivo Y81i does support the face unlock.