Unless someone has another solution, here's how I've managed to overcome the Vegas 10 64-bit version issue of not being able to view/edit AVI's.
See this thread for my issue:
You can see a link to a sample AVI if you want to try, plus the readings from GSpot and the error messages Vegas would throw up.
I discovered that the 32-bit version of Vegas 10 was able to read avi's perfectly.
So what I did is I took the aviplug.dll from the FileIO Plugins folder of the 32-bit installation and copied that across to the FileIO Plugins folder of the 64-bit install of Vegas.
All appears to be working thus far as of this post.
Hope this helps others!