No. Vivo Y81i has 2 dedicated slots for sim cards and a dedicated MicroSD card slot. You can use 2 sim cards and a memory card at the same time.
Yes, Vivo Y81i does support GPS.
Vivo Y81i is made of Aluminum Alloy and industrial grade polycarbonate material.
No. Vivo Y81i does not have the front camera flash.
Yes, Vivo Y81i does support the face unlock.