To convert VFR to CFR:
a.) use VFR aware converter like VidCoder, HandBrake or ffmpeg
b.) in AviSynth or VapourSynth make a VFR->CFR conversion. For example in AviSynth:
Code:
a=ffaudiosource("source.mkv")
v=ffvideosource("source.mkv", fpsnum=24000, fpsden=1001) # example for 24/1.001 fps ~= 23.976 fps target
AudioDub(v, a)