I've heard it said that Vrabel didn't give a clear answer as to who he would bring in to fill out his coaching staff. That was intentional. He did say that some would be people that he knows and some that he doesn't. Lord Admiral Nelson once said; "A name across the stern of a ship serves no one but the enemy." Basically, don't advertise what you're doing or give your enemy any intel.
With all of the new coaching staff's that will be filling out this off season, why would Vrabel tell anyone who he was thinking of bringing in? Because if he did that, other teams might leap in first and snag them up. I suspect that given how much time he's known he was going to be the Patriots HC, he probably already not only knows who he wants on his coaching staff, but already has agreements in place.
So, how does McCarthy fit in? He was a successful OC and was the Packers HC when Rogers was drafted. He helped develop him and oversaw a successful offense when he was HC. I wouldn't mind him as OC, so long as they bring in a solid Assistant OC Type (perhaps whoever will be the next QB Coach, which probably won't be AVP). Josh McCown is a friend of Maye, and Maye has a great deal of respect for him. McCown is considered to be a solid QB Coach and Offensive Mind as well.
On a side note. What Jones did to McCarthy was real scum baggery of the first degree. Last week he denied McCarthy a chance to interview for another HC Job. Which is fine if Jones intended to keep him as HC. But now, it seems that Jones was only going to keep him if he took a pay reduction. I'm not going to buy that Jones didn't believe that there was anyway that McCarthy would have accepted that agreement. Therefore, he hindered McCarthy intentionally. If I were an in demand HC Canidate, there's no way I'd be HC of Dallas (and that was even before what Jones just pulled on McCarthy).