Vivo Y95 does have the OTG support.
Yes, You can record slow-motion videos on Vivo Y95.
Yes, Vivo Y95 does have the portrait mode for both its cameras - rear & front.
Vivo Y95 has an aperture ratio of f/2.0 for its 20MP front camera. While the rear camera has f/2.2 & f/2.4 aperture for the 13MP & 2MP cameras respectively.