Even without an active support subscription, you are still able to get all (future) updates for the theme. And the theme author is also responsible for providing you with an updated version of Visual Composer, as you received VC bundled with your theme.
Therefore, your only point of contact here is in fact the theme author. You did NOT purchase a license for Visual Composer itself, only the right to use VC as long as you keep using the theme on the same site. That means the author of VC is in no way responsible or required to provide you with any assistance or updates directly. If you want that, you need to in fact purchase your own license of VC.
But until then, everything has to happen via the theme (author); Envato’s license and support rules are quite clear here. And as stated, even if you don’t want to pay for extended support itself (after the first 6 month after purchase), you still get all theme updates for free, provided the theme author is still maintaining and updating the theme. But at the same time, a theme author is free to “abandon” a theme at any given time and is not actually required to publish new updates, but if they do, you will have access to them through Themeforest.