Supposedly the Falcons are basically married to Vick for the next couple of years because of what trading him would do to their salary cap. Add to that what Randy Moss makes, and they couldn't afford to make that deal even if they wanted to. Besides, I can't seriously see Randy Moss playing for the Falcons as long as Blank owns them. He's already passed on both Moss and Owens once, and I'd be shocked if he just traded the Vick headache for the Moss one.
(Disclaimer: I do not pretend to understand the NFL salary cap myself; I merely have to believe & parrot what the Media Guys tell me about it.)