I suspect it's a problem with the files themselves and possibly Vegas; but since there are a number of things going on here, I would suggest starting from the beginning and re-doing everything.
If there's a chance your registry is screwed up, CCleaner
Then install programs from the latest AMVapp
Then, the obligatory "Reinstall the latest CCCP
via right-click > run as admin..."
Then, try loading the file into VirtualDub or AvsPmod through AviSynth
If that is possible, then you can convert
them to something that does work with vegas, or frameserve
And if that still doesn't work, it's likely a problem with Vegas or the files themselves;
possibly incompatible codec versions between the video files and the installed codecs on your computer.
So, IDK...maybe reinstall Vegas or new/old/different codec versions if you have to.
And if the problem is just at certain frames, you'll maybe have to split the file around the corrupted frame(s) using VirtualDub.