Vector tiling is still useful for dynamically simplifying dense maps from a single data set, so that a browser doesn't have to consume and process a few dozen MB of GeoJSON or what have you into SVG or canvas output, but TileMill isn't the tool for that either. Mapbox maintains a list of vector tile parsers and generators: https://github.com/mapbox/awesome-vector-tiles
Also, map data is getting progressively richer and more expensive to render, particularly when adding 3D data, so I'm not convinced that offloading everything to the client is still a 90% use case.
Also, map data is getting progressively richer and more expensive to render, particularly when adding 3D data, so I'm not convinced that offloading everything to the client is still a 90% use case.