Yes, you can play PUBG game on Vivo Y91 easily. Please check the following video which will give you more idea about the gaming performance on Vivo Y91.
No. Vivo Y91 is not a waterproof mobile phone.
Yes, Vivo Y91 does have the dual rear camera set up. It has 13MP + 2MP rear camera set up.
Yes, Vivo Y91 does have the VoLTE support.