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.

As a big user of vega lite I think that's fair. I think it really shines when used by data vis experts, where charts need to be precise, such as in research and analysis contexts. For something like a simple a metrics dashboard I think I'd agree that it may be difficult for devs.