If I understand correctly, you received Visual Composer bundled with your theme, and if so, you do in fact NOT have a license for Visual Composer itself. You only have a license for the theme, which includes the right to use the Visual Composer version that came with your theme as long as you are using the theme on the same site. Any support requests you might have with Visual Composer must be directed to the theme author, as the author of Visual Composer itself is not required to respond to those, as you simply don’t have an actual license for that plugin.
And if the theme has now been removed from CodeCanyon, you need to check if your purchase of the theme was during the last 6 months (or if you have any active support extensions for the theme), and if so, the theme author is still required to assist you, even if the theme is removed.
But if your purchase date is older than 6 month (and you don’t have any active support extensions purchased), you are unfortunately on your own. You now have a discontinued theme and most likely an outdated plugin (Visual Composer), as you will now neither receive any support, or updates.
In that case, one can in fact only recommend for you to purchase your own license of Visual Composer, so that you can receive plugin support and updates directly from the author of Visual Composer. That does, however, not solve your issue of eventually or already running an outdated theme, which might not be fully compatible with WP or Visual Composer anymore (down the road).
Or, alternatively, you can purchase/use a new/different theme that also includes a bundled version of Visual Composer, so you don’t have to buy VC directly, but still have access to VC updates (via the new theme) AND also an updated/compatible theme in general.
One thing to have in mind though is that if your current (discontinued) theme provided additional elements to Visual Composer that are unique to the theme, and you are using any of those, switching the theme will most likely “break”/remove those elements from your sites, and you will see a shortcode string instead.