What is the relation between the mp4v codec and the H.264 format?

Just to clear things up a bit. mp4v is not really anything. It is used in VLC to represent MPEG-4 Part 2 video, but in a technical sense is nothing.

MPEG-4 is the general name for a set of specifications defined by ISO/IEC 14496. There are several parts to this specification. These are the relevant parts:

• Part 2 - Commonly known as MPEG-4 Video, is a video compression format.
• Part 3 - Commonly known as MPEG-4 Audio, is and audio compression format.
• Part 10 - Commonly known as H264 or MPEG-4 AVC, is also a video compression format.
• Part 14 - Commonly known as MP4, is a container format.

H.264 is typically considered to be the better for video compression. It contains several features that MPEG-4 video does not including those listed here.

The biggest difference you will typically see is file size. H.264 will typically be able to compress video at a much lower bitrate than the standard MPEG-4 codec. You should be able to get comparable quality video with much smaller file sizes.

MPEG-4 specifies several different video (and audio) codecs. One video codec described is H.264, but not the only one.

So what VLC calls mp4v is probably one of MPEG-4's other video codecs. I didn't find anything about it in VLC's docs, so you'll probably have to dig into the source to find out which one.

It might be the codec described in Part 2 of the MPEG-4 spec.