VLC won't become default for MP3 or MP4
Solution 1:
Try one of these 2 solutions I hope they'll solve your problem:
1- use "Default Apps" for change every default application.
2- From the Finder, select a MP-4 file, open the Info panel (File ➔ Get Info or ⌘I), select your preferred application and then click Change All…
Solution 2:
You can use utility like Magic Launch to do that.