So Vega trades off expressiveness for interactivity, interesting perspective. But, I think this only applies when manually writing the Vega (Lite) JSON: for me an important advantage of the spec being mere JSON is that it's easy to generate visualizations from another language. This approach again limits the interactivity, but only as much as you choose to - the more you generate and pre-compute, the less interactivity is possible.

(Don't know why you're down-voted.)