Have a look at the VLC Wiki. Under the search term "vlcrc", you'll find the page called Preferences. Here:
https://wiki.videolan.org/Preferences/
Depending on your operating system, the vlcrc file is located elsewhere. For Linux, as my user name on my computer is called "Bird", it would be the path /home/bird/.config/vlc It's a hidden folder, so you probably should type it into the address bar on your Linux.
You can edit the vlcrc file with a standard text editor. Search for the line "stereo-mode". Remove the "#" in front of that line, because it would comment the line out. Maybe you've seen this in programming code. The lines with a "#" aren't used in the actual program.