Was literally just about to ask if a Python api for Vega existed when you posted!

It looks great! I can't believe I haven't heard of it before. Have used Plotly a lot in the past, this looks like a great alternative.

Just for completeness, you might be interested in the Holoviz ecosystem (core library Holoviews + additional libraries like hvPlot), which uses either Bokeh or Matplotlib as its backend.