Very different, nvd3 and pretty much all charting libraries are built around the idea of charting templates. What Vega (and its simplified cousin Vega-lite) provide is a grammar for describing the mapping between the data space and the visual space.

What this essentially means is that you are able to build your own visualisations. If you for instance check this sample[1], and in the JSON editor, inside the "encoding" section you swap x with y, and y with x, then click on the parse button, the chart will be redrawn with the x and y axis flipped.

[1] https://vega.github.io/new-editor/?mode=vega-lite&spec=bar_a...