A vertical frame is not simply a horizontal shot cropped narrow. Every part of the screen competes for limited space, including captions, buttons and platform overlays.
Place the eyes near the upper third
Keep a small amount of headroom and avoid centering the face vertically. Eye level around the upper third feels balanced and leaves usable room for captions below.
Protect the interface zones
Keep important text away from the bottom and right edges where captions and buttons often appear. Review one test frame inside the actual platform before recording a series.
Decide whether hands belong in frame
For an energetic explanation, frame from mid-torso and leave room for gestures. For a personal message, move closer and let the face carry the emotion.