The contract is a one-year deal worth $845,000.
Tom Johnson has been relatively well-traveled in his football career. His first stop was with the Indianapolis Colts, signing with them as an undrafted free agent out of Southern Mississippi after the 2006 NFL Draft. They released him at the end of training camp, but re-signed him and shipped him to play for the Cologne Centurions in NFL Europe.
He still didn't stick with the Colts, and went on to play for the Arena Football League's Philadelphia Soul and the Calgary Stampeders of the Canadian Football League before signing with the New Orleans Saints in 2011. (He doesn't have any bounty stink on him, so he's okay.) He has played for the Saints for the last three years, amassing 55 tackles, five sacks, and a forced fumble in 40 games.
There isn't a lot of risk to signing a guy like Tom Johnson for the Vikings. He might stick as a back-up, and if he doesn't there's no harm done.