Well, we don't have any available codecs or documentation on it and noone would reverse-engineer it. But there are some other ways, right?
We do know emulators play ActImagine videos fine, let that be our starting point.
The most obvious way would be just use some screen-capture and sound-recording software, but that's not creative and movie should be accessible in game to record it so let's find another way...
At first my plan was like this:
1) We get the .vx file we want to convert
2) We get some game that starts from video (Nobunaga no yabou starts with Koei logo so we will use this game ROM as base)
3) Using ndstool we replace opening movie file (koeilogo.vx) with our file
4) Now we will need some way to record playback. Say we are not lazy (even though I am) and can add video recording o some open-source emulator (DeSmuME for example) - at least execute screenshot routine every frame - and then cobine tham into movie using side software. Well direct video output sure is better.
5) and now we launch reassembled rom in our new emulator until movie end - voila, we have a video file we need. Now we just write batch to all these actions...
Everything seems so bright and easy so why not?.. And here comes obstacle I could not overcome (was too lazy perhaps) and thus ask for your assistance. See, we caan get video using that method alriight, it played Time Hollow opening and Kirby Super Star Ultra videos but... Yeah, some of you know the trick already, audio is not stored in .vx file. It's a part of sound_data.sdat (I am not sure on this but seems most likely) and simply replacing file does not help
And here goes the question - is it possible to replace opening audio too and if it is, can it be made automatic?..
Sorry for my broken english etc, hope I said anything useful `^^
We do know emulators play ActImagine videos fine, let that be our starting point.
The most obvious way would be just use some screen-capture and sound-recording software, but that's not creative and movie should be accessible in game to record it so let's find another way...
At first my plan was like this:
1) We get the .vx file we want to convert
2) We get some game that starts from video (Nobunaga no yabou starts with Koei logo so we will use this game ROM as base)
3) Using ndstool we replace opening movie file (koeilogo.vx) with our file
4) Now we will need some way to record playback. Say we are not lazy (even though I am) and can add video recording o some open-source emulator (DeSmuME for example) - at least execute screenshot routine every frame - and then cobine tham into movie using side software. Well direct video output sure is better.
5) and now we launch reassembled rom in our new emulator until movie end - voila, we have a video file we need. Now we just write batch to all these actions...
Everything seems so bright and easy so why not?.. And here comes obstacle I could not overcome (was too lazy perhaps) and thus ask for your assistance. See, we caan get video using that method alriight, it played Time Hollow opening and Kirby Super Star Ultra videos but... Yeah, some of you know the trick already, audio is not stored in .vx file. It's a part of sound_data.sdat (I am not sure on this but seems most likely) and simply replacing file does not help
Sorry for my broken english etc, hope I said anything useful `^^
Or others 
. Saves on time, soap, water and money having to wash them.