No chance I swap Johnson for Vick AND a 4-round downgrade. I wouldn't do that for any of your backs, to be honest. I'd consider selling DMC and your 2nd for Vick and a 4th, and that's about as low as I'd go.
Remember, this guy is probably desperate for RB and wants to offload the risk of Vick, especially in a keep 3. He's the one that's pressing, so make sure you respond accordingly. Offload some risk and upgrade your QB without losing much of a pick? Sure. Just don't do that with Johnson or Charles.
With those three backs and 3 picks in the first two rounds you're set up very nicely and don't have to make any moves that aren't no-brainer upgrades for you - keep that in mind, don't take any wild deals and you'll be golden after the draft's 3rd round by going BPA/WR early and often