I've tried to convert a lossless encoded FFV1 file to MP4 in Virtualdub, using Koepi's XviD Windows codec package build.
Regardless which profile I selected or xvid settings I chose, I always got the same error when trying to start the transcoding:
[SOLUTION]Unable to initialize the output video codec. Check that the video codec is compatible with the output video frame size and that the settings are correct, or try a different one.
The output color depth (aka "pix_fmt") was set to "4:2:2 planaer YCbCr (YV16)". This actually made sense, since the input pix_fmt is "yuv422p".
For encoding to XviD in Virtualdub however, I had to set it to "4:2:2 >CbCr (UYVY)" in order to work.