I'm assuming your VOB is 23.976fps, with 3:2 "soft" pulldown making the displayed framerate up to 29.97fps.
VirtualDubMod has not been updated for a while now, and on some occasions it can fail to recognise when a video is 23.976fps with pulldown flags.
You could try VirtualDub-MPEG2 instead, it has an option to disable pulldown.
However, if the pulldown pattern is irregular, you could end up with audio sync drift. The safest method - a little more work but worth learning in the long run - is to go DGIndex "force film" -> AviSynth script -> regular VirtualDub -> save AVI.