We're not exchanging Vrabel for James Sanders. Don't think of it that way.
First, we had too many LBs on our roster (Vrabel, Thomas, Mayo, Bruschi, Woods, Crable, Guyton, Banta-Cain, Redd, Alexander, Robertson, Craig and Ruud) and too few safeties (Meriweather and Spann). We need to plug holes at safety, and we have room to spare at LB.
Second, you need to look at the who roster picture. Many people were suggesting that Vrabel could be a cap cut, or at least needed to restructure in order to not be a drain on the roster. We essentially cut him, but allowed him to save face by being a veteran leader under Pioli in KC. We need his cap savings to build our roster, and Sanders is just a cost-effective piece of that total picture, nothing more. $3M/3 years is a very reasonable contract for Sanders. I'd much rather do that then spend $5-6M on Sean Jones. Miami gave Gibril Wilson $27.5M over 5 years. I'd much rather have Wilson than Sanders, but not if the added money cuts in to quality signings at CB or LB, or major resignings.
If anything, you can look at it like we picked up Tully Banta-Cain for a vet minimum instead of Vrabel for $4.3M. While I'd take Vrabel any day, the cost savings is significant, and if you believe that Vrabel was only going to be a role player and that the young LBs needed to step up and take over then it just wasn't worth it. As a 4th OLB who can rush the passer, Banta-Cain will probably provide similar value to what we would have gotten out of Vrabel, aside form his leadership intangibles. Those are important, but at some point we have to move on and hand over the leadership to Wilfork, Thomas, Mayo and Meriweather. Those are our core defensive leaders right now, not Vrabel, Bruschi or Harrison.