“There is a second more complicated way to convert your RMVB files. VirtualDubMod, which supports AVISynth scripts and MPEG-2 video importing, is an alternative method for converting RMVB files to a format suitable for DVD.”
According to Wilbert on the VideoHelp.com forum, you need the latest AviSynth v2.56 for the convertfps to work. That did the trick for me, as I was first having trouble with VirtualDubMod giving me DirectShowSource does not have a named argument “convertfps” errors when trying to load the Realmedia file.