If you have a dead side when connected with a cable, you’re going to have to open them up. A solder connection probably failed. EPA and EU approved lead-free solder can fail for no real reason at all. Premium products like V-Moda usually use high silver content solder that can be just as good as the lead doped stuff, but who’s to say their factory partner didn’t cheap out for a run.
If it works fine when wired, but there’s a dead side with Bluetooth there are some possible easy fixes. First, try to reset the firmware by turning the headphones on, make sure nothing is connected, then hold the volume up and down buttons for at least 3 seconds until you hear a tone. If that doesn’t work, start playing something over Bluetooth, and while it is still playing connect an audio cable (not charging) that isn’t connected to anything on the other end. Leave it connected just long enough for the sound to stop playing, then take the cable out. Credit to iFixit user Rohith for that last solution. If that still doesn’t work, it’s probably a failed solder connection.