I'm not sure what you are trying to do with virtualdub so I'm not sure how to try to help but,
here are my thoughts.There is a tweaked version called nandub which will open some files that
virtualdub doesn't like, maybe try an older version of virtualdub (you will have to look around for one)
some of the really older versions open file types that the later ones won't. With video codecs always seem to
be a problem for some people, but I don't think that should be the case with a mpg. There is info and a forum
at
http://www.virtualdub.org/ where you might find what you need to know. Finally in my experience there are some vids that vd just doesn't like. For these I usually re-encode them or convert the vid with some other program.
Hope you can find some help somewhere in there