The grammar of Vega is a description of visualizations, but it isn't a grammar for computation of visualizations. That's where these tools fall short for me, in the sense that they fall short of VTK, VisTrails, and even old OpenDX. I appreciate a regular description, but it seems relatively powerless for data exploration where respect for the model, for uncertainty in data, for symmetries in data, ask for customization of visualizations. Vega is a nice piece of work, and I'd like more, much more.