Two issues, first is that firefox no longer seems to allow file:///tmp/plot/hp-mpg.html be opened as configured by default on Ubuntu. You need to move the file to ~/Documents to open it, or do some other configuration magic which I can't figure out.
Second, the example (image below) has (vega:defplot hp-mpg ... :data (:values ,vgcars)...) which seems to no longer be the usage --- this dies with column-length being called on a data-frame. Removing the :values and just doing :data vgcars lets it work.
