Yes, Vivo Y81i does have a dedicated MicroSD card slot.
Vivo Y81i has FunTouch OS which is based on Android 8.1.
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.
No. Vivo Y81i does not have the removable battery.