Visual Composer Website Builder has a Free version that can be included in a theme completely free of charge. Your theme users will need to activate the free version to design their pages.
You can include custom content elements with your theme with help of extensive API which allows to develop custom content elements and bundle them into a theme. Custom elements will also work in the Free version so your theme users will be able to use them right away.
Read Theme Integration announcement for inspiration and to learn how to increase your theme sales.