Install AviSynth (http://avisynth.org/mediawiki/Main_Page)
and AVS2AVI (http://hmd.c58.ru/files/avs2avi-0.3.zip).

Create a text file with

AVISource("c:\location of file.avi")
ConvertToYV12()

and save and change extension to .avs (use a different name than your original file, otherwise it won't work.)

Right click and select "Wrap Into AVI". A DOS window pops up and gives you a little information. Press a key.

Now load the fake AVI into Vegas. Note that this won't load the audio portion, but that can come from the original AVI file. SD works with just a little lag. HD is quite slow, but manageable.

I hope this helps somebody.