As an aside, wikimedia is switching all their chart support from vega to echarts.

I'm not an expert on either, but my impression is that vega tries to do literally everything using a custom json syntax which can get confusing and unwieldly. Echarts is a bit more straightforward at least for the simple case.

Vega has a questionable security track record which is concerning if you let untrusted users define graphs.

We use Vega pretty heavily, its a broader ecosystem. Where it really shines is in combination with Altair and Vegafusion to do number-crunching on the backend and return a chart spec that can just be rendered on the front-end.

That makes it particularly useful when building interactive visualizations with a lot of data.