My MythTV records over the air broadcasts as .mpg (MPEG) files of several gigabytes each. My preferred mode of viewing them these days is to open up a SMB share on the MythTV box, copy the file over to my MacBook while I'm working and then view it later. Amongst the many annoying things about this process—inefficient SMB transfers, avoiding having Time Machine back up the show, OS X not letting me watch a partially transferred file—a small one is that Quicktime owns the .mpg extension, when it'd be nice if VLC did. But every time I would open a Get Info window on a .mpg file the whole Finder would lockup, apparently generating the preview. And then I'd relaunch the Finder after a minute or so. So I never was able to click the Change All... button.
Turns out that the Finder will eventually, after several minutes, give up with creating a preview, and you can change the Open With application to VLC and then click the Change All... button. So be patient.