encode variable frame rate mkv

90% of my encodes are anime and I was wondering if there is an avisynth plugin that will enable me to encode my mkv files using a variable frame rate. I would like to do something like encode high motions portions of the movie in 24 fps but lower the fps to 8 fps in portions of the movie that are still or have little to no motion. Is there an avisynth plugin that can do this? Thanks.

And are you sure that your playing device allows VFR?

I don't know about that Stephen. I admit I know very little about subtitling and its many intricacies, but I could have sworn displaying a subtitle by time range has always been one way to sync, although perhaps more difficult? Most utilities I have ever played around with, always work by using time codes and not frames as the sync point. Otherwise, how would Dual-Audio w/Subtitle VFR Anime that exists out there, be able to work (pretty much flawlessly) in the first place? I think it is more a question of patience and skill than anything else.