Hello georgiabyte,
Thanks for reaching out using Apple Support Communities!
We understand you want to change the default video player to the VLC app on your iPad mini.
There isn't a direct option to set the default video player on iOS.
We welcome and encourage you to submit this feedback and any other feature requests here: Product Feedback - Apple
You might reach out to VLC support to see if there is a similar setting from within the app.
This link can help you learn more about using the Photos app, if you'd like: View photos and videos in the Photos app on iPad - Apple Support
We hope this helps
Cheers!