Help ... I don't get it! Why is it ironic?

The Dilbert strip in the paper on June 24, 2009, features a tax auditor visiting Dogbert, CEO. He tells Dogbert, "I look for reasons to transfer money from small, disreputable entities to a larger one." (Yes, that in itself is kind of funny, but then in the volgende panel:) He then goes on to say, "I call my salary vigorish, but it's not as ironic as it once was."

Why exactly did that use to be ironic, and why is it now less so? I do know what "vigorish" means, but I still don't get the joke.