Watch and understand this first off.
https://www.youtube.com/watch?v=GhWki9a7s18
You did the exact wrong thing by just installing a shit ton of codecs. If the software fails to read audio/video content of a file, you should ideally install that one single missing codec manually. I'm suggesting you to remove all recently installed codec(s) & Vegas. Get MediaInfo and check the used codec of the AVI's video content. Then install that single codec and reinstall Vegas afterwards.
I have seen the video by now , But still do not understand 100% what the word codec itself means though. Also im really bad at fixing problems with this codec stuff, So if you can help me with that i would really appriciate that and pay you off with credits. If you like to though i hope someone can help me out with this, because im kind of frustrated, because i can't edit nor render anything anymore. If you want to help me out Rusch send me a pm and we have further contact. I'd suggest if you're going to help teamviewer i hope you will help me with this because im like totaly frustrated.