All my experience is based on a Fire TV stick however....
Vimu is basically similar to VLC but has an additional engine built in allowing it to support other formats which can be enabled and disabled within its settings. VLC on Android TV (and variants) does not display image files for example JPEGs as it simply does not have the function for images/pictures (it only does audio and video on android TV). That and a few interface differences is about the only real difference. VLC on Android TV is not the same it should be noted as on regular Android.
If it were a case of as you state and the google play site states "play the formats supported by the device running it" (by the way the vimu site is vastly out of date if you look at
www.vimu.tv/blog section it only mentions upto version 7 they are on version 8 of the app now that description on there and google play is old) then people running kodi on a fire stick or some android/TV/devices in general will be screwed. If you look at the vast amount of formats in the kodi.conf file, you will see there are plenty of formats in that where it may be a case of kodi runs on certain devices (both android and some TVs) but the android/TV/device does not even support half those. Hell if someone has kodi on a 10+ year old PC it will not playback half the formats perfectly because the hardware will not be up to it.
In most cases though they will all play fine because kodi like VLC and Vimu know what to do with the file, the only time it matters will be if someone has a crap device with a slow processor and they try playing lets say a 8k file. Kodi, vlc, vimu will all try to play it but if the device is not up to the task it will either just result in stuttering playback of other weird video artifacting. Limiting the kodi file to basic file types just like with vimu would be daft, people running any app on hardware not up to the task of playing extreme file types are to be blunt not very smart and those that have a device capable should be able to experience everything not only the device can do but the software also and not suffer.
Kodi, Vimu and VLC use their own engines to identify file types
AFAIK.......... The device it is running on should not make any difference (well maybe if its a very old Android version it will matter or if the device has a poor performing old processor, but in that case transcoding may hurt it more anyway). If it did make a difference then AVI, xvid, divx files as an example should not play on a Fire stick without transcoding because by default the amazon fire stick is only capable of the following formats...
Video formats: H.264 1080p30, H.265 1080p30
Audio formats: AAC-LC, HE-AACv1 (AAC+), HE-AACv2 (eAAC+), AC3 (Dolby Digital), eAC3 (Dolby Digital Plus), FLAC, MIDI, MP3, PCM/Wave, Vorbis, AMR-NB, AMR-WB, Dolby Audio, 5.1 surround sound, 2ch stereo, and HDMI audio pass through up to 7.1
The 4k fire stick is the same except obviously video files can be up to 4k resolution.
HOWEVER in vimu, vlc and kodi on a fire stick ALL divx and Xvid files will play fine
NO TRANSCODING NEEDED, despite the fire stick specs saying it will not do it, you only have to tell the conf file to support avi format and it will do anything in avi format. You do not need to specify the codec as Vimu, vlc, kodi takes care of and recognises that in its own engine.
Running Vimu on an older version of UMS (IE 9.4.3 and earlier) worked just fine, it just results in it using the Google-Android.conf anyway. That conf file has no info in it for subtitles yet subtitles will still work perfectly fine without the settings which have been added in 9.5.0 and the Amazon-FireTVStick-VimuPlayer.conf file.
About the only file type you had to use the transcoding folder for in 9.4.3 and earlier what i could find was FLV files and that is only because the Google-Android.conf and its line of StreamExtensions = did not have flv in it (which correct me if im wrong but they would playback on any android device anyway also???) modifying the Google-Android.conf file to include flv in that line results in flv files also working fine in Vimu.
If you really want to define a conf file for vimu which has the sections of # Supported video formats:, # Supported audio formats: etc then the kodi file may be the best place to start as basically anything already mentioned in that should be capable in Vimu
AFAIK though obviously i can not guarantee it as normally when i am using my fire tv device i do not use it with UMS, even though it works.
Personally i think for all the android based conf files, the kodi ones, the vlc one and now the vimu one they would all be much more simple to maintain using the StreamExtensions = line in a conf. If not then VLC technically needs another conf file from what i can see as technically speaking the VLC-for-desktop.conf file is not the same as what VLC on SOME android and other devices is.
Either way whatever the decision (its upto you clever guys
) this was just my opinion, keep up the good work and either way its nice to see the Fire Stick and Vimu getting some love
EDIT:
Funnily after digging a bit further it seems my ethos/thinking was the same as someone which previously did older conf files. If you look at the XBMC.conf file (an older version so to speak of kodi) that uses the StreamExtensions = technique/method. So i have no idea why anyone went to the bother of manually defining nearly every format in the world for the kodi.conf file
sure it is more complete but they could had just done the same/similar to the xbmc.conf file and added additional formats they wanted to a StreamExtensions = line atleast thats what i would had done. Ah well different strokes for different folks i guess some just like hard work