@ VirginiaCentral: According to Wikipedia, in the United States, the word "vigorish" also means the interest on a loan paid back to a loan shark ("a person or body that offers unsecured loans at high interest rates to individuals, often backed by blackmail or threats of violence").
The implication is that the tax system is a kind of loan shark ("give us your money or we'll lock you up" sounds like blackmail), and since the tax-man takes a salary out of taxes (in a sense, he is taking a cut), his salary could be analogous to a vigorish, or at least a portion of it.
I guess I'd have found this funnier if I'd known these terms right off the bat instead of having to look them up and then piece the gag together. :-)
I don't get it. I looked up "vigorish".
1: a charge taken (as by a bookie or a gambling house) on bets ; also : the degree of such a charge <a vigorish of five percent>
2: interest paid to a moneylender
That didn't help. I guess the thought of the IRS taking all of your money is neither ironic nor funny to me.