Venus outweighs its atmosphere by about 10,000 times. This is actually less than I thought -- for comparison Earth outweighs its atmosphere by over 1,000,000 times, which is still far less than I would have guessed.

Venus's escape velocity is less than 1/3rd of its orbital velocity. According to google, Venus's orbit, despite being very circular, causes its velocity to vary by a KM/s from aphelion to perihelion.

So I believe you could send all of Venus's atmosphere off permanently into space at the cost of about 1/30,000th of Venus's orbital velocity, meaning you could very slightly circularize its orbit further.