Yes, Vivo Y81i does support the face unlock.
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.
You can record up to 1080p resolution videos on Vivo Y81i. You cannot record 4K videos via Vivo Y81i.
Vivo Y81i has 2GB RAM of LPDDR3 type.
Yes, Vivo Y81i does have a dedicated MicroSD card slot.