You cannot demux video at different frame rates. Demuxing just extracts the video elementary stream from the program or transport stream. It will be 29.97fps because that is what DVDs specify. But if you use VirtualDubMod, it will throw away repeat flags and you'll end up with 23.976fps if the source uses soft pulldown. Try loading your source directly into VirtualDub MPEG2 with the 'Options/Preferences/MPEG/Honor repeat-first-field flags' box checked. Then you will have 29.97fps, but with combed frames if there is pulldown. You can deinterlace those if you want progressive output. If your source really is 23.976 progressive, it's doubtful you'll improve it this way, however. There really is no shortcut to learning about video. Getting the best treatment depends on so many details that you have not told us.