select one of your .avi files and right click and select "get info." Then go down to where it says "open with" and change that to VLC. Then click on the button below that "change all." This will make VLC the default app for .avi files
highlight a .avi file (or anythingelse you want to change), then control click or command + I, when the info box comes up, under Opens With, change to VLC and Change All.
I'm new to Mac and couldn't work out how to change the default video opening application. I wasted an hour trying different things, but when I did arrive at the Mac forums and read your advice, it was so easy. Thank you, you're legends.