No. Vivo Y91 does not have the front camera flash but it does have the display flash. Which helps to capture good selfies in low light conditions.
Yes, Vivo Y91 does have the OTG support.
Yes, Vivo Y91 does support the face unlock. It also has a fingerprint scanner.
Yes, Vivo Y91 does have the dual rear camera set up. It has 13MP + 2MP rear camera set up.