Mitchell was mainly a special teamer for the Dolphins in 2011, but he played enough defense to have a couple of passes defended and a pretty impressive interception against the New York Jets in the 2011 regular season finale. He also forced two fumbles last season, and has four forced fumbles in his NFL career.
Prior to playing with the Dolphins, he spent four years with the New Orleans Saints* as the team's seventh-round pick in the 2007 NFL Draft. Hopefully a year on South Beach was enough to wash the stench off.
So, we have another signing that is supposed to add "depth" to the Minnesota Vikings. I'm not sure how much time Mitchell would see at linebacker, but he should be a pretty solid addition to the special teams units for this football team.