Since (happily) installing Zorin OS 16.2, I've installed additional software for differing tasks. For example, Shotcut for video editing, and VLC for media.
I have noticed Shotcut is registered in Zorin for watching media. I didn't want this, so I went to default applications in Settings to change it to VLC. Unfortunately VLC was already set as the default app. Changing the default between Shotcut and back to VLC did not change the registration, and I have to force mp4 files to open in VLC. Below illustrates this predicament.
Thank you for the link, but my first post's screenshots are the same attempts as described in the other forum post.
Admittedly I installed VLC via snap, but Zorin's APM shows VLC installed. I could try uninstalling and re-installing with Zorin's package manager, but I'm hesitant as the repo version is 3.0.9, versus 3.1.8 by other sources.
Snap versions are usually the latest, but due to "sandboxing" employed by Snap's, they can exhibit functionality limitations with rest of the system. I avoid Snaps and Flatpaks myself and keep to apt's even if older versions. You may also have a problem if you have two versions of VLC installed at same time.
Please try right click-->properties-->open with-->select VLC(which will lead to 'set as default' option being visible)-->set as default
Doing this would lead to that default being set for all mp4(or any other file type) files
I reckon the default app setting problem may be due to you having the Snap version of VLC not apt version.
If it was me, I would uninstall the Snap version and choose the apt version from the Software store.