Quicktime X does not want to play certain MPG files. How do I change the default to be VLC?
If you right-click on the file and select 'Get Info' (or press ⌘ + I when the file is selected in the Finder), you can expand the 'Open With' section, pick an application, and click 'Change All'. It will pop up a confirmation box to which you will click continue:
This should change the opening program for all files with that extension. Cheers!