The reason why this is not working is because you are trying to embed VC inside the lightbox modal.
VC shortcodes would work inside the post/pages of wordpress but not on modals.
I don’t know if a paid add-on plugin called “Modal Popup Box For Visual Composer” would help you. It gives yous the ability to make vc content appear on modals but I’m not sure it would work inside the lightbox. (I also don’t know which lightbox plugin you use, they are so many).
Hope this helps