Couldn't we just build a MegaMaid and suck the atmosphere out? If we're going to go sci-fi, it seem easier to hoover it out than capture an asteroid and nick the atmosphere just right.
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.