I think I have a similar problem.
I bought a theme (WPLMS Learning Management System) and when I was trying to update Visual Composer it asked me to put license key, but I don't have it, or don't know where to get it?
I found where you can purchase it from, but from what I understand from the posts above the license should have come with the purchase of the theme?
From the download files (the theme's files) there is a "EvatoMarket License Certificate" file in which there is an item "Item Purchase Code", now this code looks similar to what a Visual Composer license key should look like, but even if I use it it does not work ...
Any help will be greatly appreciated!