It doesn't work for everyone, but my experience is this:
Jenny was vaccinated with Case-Bac. Her dam didn't have CL, but a couple other does did. Whether she was exposed to an abscess or not is unknown, but odds are no. She has yet to break out with an abscess, but from what I know, because of the Case-Bac, I cannot blood test her for CL. I can only check her over thoroughly for abscesses and be happy that she has none.
If you want to run the risk, and you think they are worth it, go for it. If they have been vaccinated, there's a fair chance they won't ever get an abscess, but there's always the risk that they will, regardless.