Stream #0:1: Audio: ac3, 48000 Hz, stereo, 256 kb/s
You are trying to mux an AC3 stream into an MPEG container. You cannot do this. You could use a different container
ffmpeg -i file.vob -c copy out.mkv
or perhaps allow FFmpeg to transcode the audio
ffmpeg -i file.vob -c:v copy out.mpg