On the latest version of VRChat, if you open the streaming camera, turn it to portrait mode and then enable the SPOUT stream it will rotate and stretch the video in the camera display. The only way to get it to display correctly is to enable SPOUT while the camera is in landscape mode and then rotate after SPOUT has been enabled. I've attached a video demonstrating this behaviour.

I have only tested this on PC (confirmed by 2 friends who have the same bug)