Did you use the manual setting for VirtualDub, or the "Adaptive" setting? The adaptive setting can ocasionally lose its pacing, even if there isn't a cadence change. The manual setting is much better, but if there's a cadence change, you have to find that spot and restart at that point. It sounds tedious, but I found the cadence change in about 5 minutes (I viewed the AVI in Vegas until I saw bad combing effects.) After that, you try different offsets and watch the first few seconds of your output to see if you got it right.
"it is strange that we don't seem to be able to come up with a definitive way to IVTC the capture."
Good IVTC is highly dependant on the telecine process that was used in the first place. It's also dependant on the quality of your capture.