I've used both, and while I think Vega has it's uses, it's not nearly as web developer friendly. Frontend engineers want a clear delineation between logic, composition and styling. By combining everything into a JSON document, you sacrifice that developer experience while introducing a lot of bespoke approaches.

That said, I absolutely love the idea that a blob of JSON living in my database contains everything I need for my visualization. The reality is that not enough other people are willing to put in the effort to learn that syntax, making it somewhat of a selfish tech choice.