Vegas pro will not show video for avi with mp4v fourcc codec?
I resolved this issue by installing VLC. Apparently VLC has some necessary codecs that Vegas Pro uses.

After installing VLC I was able to import the video and use it normally with no hindrances. 



After some time, this issue cropped up on a different computer. Installing/reinstalling VLC did not work.

I installed AVI Codec Pack Pro v2.4.0 from Softpedia and the issue was resolved. 

If Softpedia no longer has the file available by the time you are reading this, the file was titled "avi.codec.pack.pro.v2.4.0.setup.exe" and was 16.7MB (17,538,685 bytes). A google search at the time of update yields other sources for download, but some are concerned with malware imbedded in it. I haven't noticed any malware with the softpedia download.