What you need to do is convert that xvid to a format Vegas will recognize. What i did was use DVD architect pro. I loaded xvids on to DVDA and then made a dvd. I then took the vobs DVDA outputted and used them in vegas using the audio cut from the xvid.
Vegas does not play well with XViD and DviX. Yes, Pro is a bit better, although sometimes you see that some files don't have audio, while if you open them together with other files, the audio is recognized. Weird stuff. And even when you get it loaded, if you put it in the timeline, I had instances where big pieces of a 5 minute clip would not show up in the preview window and it will look all red in the timeline, while the rest of the same clip worked fine. Weird stuff No2.
Vegas Platinum is even worse. Much worse. I had very few XViD files recognized.
And I have tried ffdshow's brand new codecs, and xvid's codecs and dvix's codecs. Vegas recognizes none on my machine perfectly and it either doesn't work at all with these files or it has major problems.
So, get these files, download mencoder or ffmpeg to save on a lossless codec like lagarith or huffyuv, and then import them to vegas. I can help you if you want with these command line utilities.