Yes, Vivo Y81i does have the virtual gyroscope sensor.
No. Vivo Y81i does not come with the earphone in the box. You have to purchase it separately.
You can record up to 1080p resolution videos on Vivo Y81i. You cannot record 4K videos via Vivo Y81i.
Yes, Vivo Y81i does have the OTG support.
Vivo Y81i has FunTouch OS which is based on Android 8.1.