None of your QBs are good enough this year to wring your hands over. I could dump any 2 of them and not lose any sleep. Vick's season doesn't look very promising and Dilfer is unlikely to stay healthy.
1) Definitely get Maddox.
2) Your record, and the relative strength of your opponents should decide who you unload. If you're doing well, drop the QB who has been least productive. If you have a losing record, keep the guys with the most upside, because you will have to play a riskier game to compete.
I really like Maddox, and you should too. He plays alot of weak defenses which, win or lose, will give him great numbers. Look at his XFL (where he was league MVP) and Arena Football numbers and judge for yourself.