All right, I'm not sure if anyone cares about the details -- I'm not sure I do anymore, and I'm a Falcons fan -- but here's how they apparently got to 23 months. Vick was originally supposed to get 12-15 months, per his plea bargain. But after the failed weed test, the judge had to bump it up to the next range, which is 18 to 24 months. So he went back to the US attorney to ask for a revised recommendation within that range, and the US attorney said, "Based on the fact that we don't believe he's cooperating like he promised to" -- among other things, Vick supposedly failed a polygraph test w/r/t the question of whether he personally killed dogs -- "then we recommend that he be given a sentence towards to the top end of that higher range." Thus 23 months.
(I heard it on the radio, so it must be true.)