The VideoLAN Forums

Is it possible to set all videos to 16:9 aspect ratio permanently via padding? (Adding black bars)

Yes, either with crop or aspect ratio preference parameters, depending if you want to crop or distort.

I'm interested in padding, not cropping or distortion.

VLC automatically pads to fit the window. There's no particular setting needed then.

I'm using a large VLC window and interested in padding ultrawide videos in order to get 16:9 aspect ratio. (black bars above and below.)

From a logical standpoint, the only ways to change the aspect ratio of the rectangular video are to crop or to distort.

VLC can't invent video pixels that don't exist in the source material.