Thanks 6_pac for bringing this post back up. I saw it many hours ago but got side-tracked despite I had started to answer it. Well I have QT as default for some formats & VLC as default for all the rest. To set default in OSX there's more than one way. Select the video file, go to File>Get Info (⌘ + i ) & choose application to open it. You will see you can also set that app as the default. Another way is to control-click the file.

Personally, I go directly to the incomplete folder to view directly so I don't need to go back & remove preview files later. You can just drag & drop the file onto VLC. Actually VLC handles incomplete files much better than QT does. Off-hand I can't remember which format(s) I have QT as the default. I think Mpeg2 is one b/c I have the mpeg2 playback plug-in for QT. Most of the others I use VLC for.