The answer is licensing and patents.
Dolby Laboratories holds patents over AC3 and E-AC3. Distributing software that includes these decoders without paying a royalty fee is illegal in most jurisdictions. Since MX Player is a free app (with ads), the developers cannot afford to pay per-device licensing fees.
By offering VMD as a separate, optional download, MX Player shifts the legal responsibility to the user. It becomes a "neutral tool" that can play AC3 if the user installs the plugin themselves. vmd codec for mx player
If you have a file ending in .vmd that nothing else will play, MX Player combined with a Custom Codec is the best solution on Android. It turns a "corrupted" file into a watchable video.
However, for everyone else, avoid the headache and stick to the standard MX Player settings. The answer is licensing and patents
The VMD codec is a video/audio codec format used by certain cameras and multimedia devices. MX Player is a widely used Android media player known for supporting a large variety of container formats and codecs via built-in decoders and by allowing third-party codec packs. This essay explains what the VMD codec is, why MX Player may need an external codec to play VMD files, and how users can enable playback.
If installing the custom codec (sometimes wrongly called VMD codec) doesn’t solve the problem, try these alternatives: The VMD codec is a video/audio codec format
VMD is a multimedia container format developed by Sierra On-Line (later Sierra Entertainment). It was prominently used in the late 1990s and early 2000s for video games on platforms such as Windows, PlayStation, and Sega Saturn.
Unlike modern codecs (H.264, HEVC) designed for high-definition streaming, VMD was engineered for the constraints of CD-ROM drives and early PC hardware.