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.
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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.