Lately I've been playing with VJoy to get all my Steam fighter games working with my Xtension arcade cabinet (the controls output keyboard button presses). I came up with this janky solution to start VJoy up when you run a specific game and then kill it when you're done playing. I hope this helps someone. Just be sure to leave one of your controller buttons (for me the EXIT button, which is ESC on the keyboard) unassigned to Vjoy so you can push that to unpause the batch file (the rest of your buttons won't be able to interact with the pause press any key if you select the block key function on Vjoy).
This assumes you've already installed and set up vjoy to play with certain games and it's annoying to leave on because you have it blocking keypresses. You want to automate things from BigBox.
Create a folder c:\vjoybatch
Create a batch file in there called vjoybatch.bat and put the following in it (may need to change the VJoy path name depending on where you installed it and the steam path name depending on where you installed that):
echo Starting VJoy to emulate virtual joysticks...
cd "c:\program files (x86)\VJoy"
Start /MIN VJoy
echo Running Steam game...
echo After the game is done, please press EXIT
taskkill /im vjoy.exe /f
Now in Launchbox, go to the Steam game you want to run with VJoy. Edit it and in the Emulation tab Add a new Emulator called VJoyBatch. Emulator name: VJoyBatch. In Emulator Application path browse for c:\vjoybatch\vjoybatch.bat. Click Don't use quotes, use file name only. Under associated platforms put in Windows.
In the Launcher tab under ROM file, erase "steam://rungameid/" and leave the trailing number there; that's your steam game ID.
Now the last thing you have to do is go to your c:\users\(username)\launchbox\ folder and create an empty file named your game ID. For instance, say the game ID is 35321. The ROM file in the Launcher tab should simply say 35321 and in the Launchbox folder create a blank file named 35321 (make sure you don't make it 35321.txt; it should just be plain old 35321). If you don't do this step bigbox/launchbox will complain that it can't find the ROM file.
After you do all this stuff, when you run this specific game from BigBox, the batch file with run VJoy, run the steam game, then pause for input. After you've finished playing the game, push any key (that isn't bound to your vjoystick) to unpause and the batch file will kill VJoy and you will return to BigBox. The little joystick icon may still show briefly in the bottom right toolbox but it will disappear.