Put them both on a 24p timeline and render. Vegas will do no-recompress pass-through of the frames, so there will be no quality loss. You can verify this by watching the Video Preview window during the render -- no-recompress frames are not shown, so if you only see black during the render, all is good.
Can you do this with mpeg 2 files? Can you also trim start and end points on an avi without recompression? Is there somewhere I can go to read more about this subject?
No, you cannot avoid recompressing with mpegs. Vegas is designed to edit DV, so the format internally is DV - hence no recompression. An AVI is just a container, it depends on what's in the AVI as to whether it will need to be decompressed or not.
There are some mpeg-2 editors available - www.womble.com for example.
I've never used it, so I can't recommend it. However other folks here on the board have used it and it seems to be the one that gets refered to most frequently.